Games without violence

Kerbal Space Program - lots of explosions though
ABZU - maybe one little part might be considered violent but its mostly underwater diving and exploration
card games, maybe?

do you require the games to be coop?

Single-player, there is quite a bunch of non-violent games. Multiplayer ones are more limited in numbers, but there are some too. Besides racing games (such as Dirt, Grid, Trackmania, ...) there are e.g..:

Offworld Trading Company (has sabotage actions, but pretty non-violent)
TransOcean 2: Rivals
Pool Nation
Football Manager (which takes quite some time tho)
Robot Roller-Derby Disco Dodgeball (which has knocking out, but everyone plays as a robot)
Tropico 5 (which has violent options, but possible to play multiplayer against each other competing for most income or buildings without any violence - and currently in a Humble Bundle)
There are also some card games, about which one could argue that not really depicting violence when one card "trumps" another, such as e.g. Spellweaver
And if building stuff together is cool or even better instead of competing, there is e.g. Space Engineers and Rising World (which have violent options, but possible to play without)

I think Obduction should count, and I love it and highly recommend it if you & your friend like solving mind-bending puzzles. The backing premise/story includes some 'threat' of violence/destruction, but for the most part the game is nonviolent... Oh, and it's in the current Humble Jumbo Bundle 11, but you can get it on sale for ~$7 sometimes too...

A lot of point and click adventures should be non violent.
Then there's submerged, Stardew Valley, Yonder, Event [0] is, as far as I remember, non violent ^^ There are many great games :3

Astroneer

A lot of games don't have violence.
But then again it all depends on your definition of violence.
A game like Mario Kart or Mario can be considered to have violence.
One could argue that even racing games have violence if cars can touch each other.

But here are games that I certify 100% violence-free:
The Witness - Top-tier puzzle game
Planet Coaster - Even crashing a coaster through a crowd just pushes people around. It gives the best experience and feel of riding a coaster that I have ever had.
FEZ - A puzzle game based on perspective with a charming atmosphere and really challenging problems.
Scanner Sombre - An exploration game with a really visually stunning concept.
Tengami - An atmospheric zen puzzler with a nice adventure and visual style.
Track Mania United - There is only you on the track, doing aerials and going through really fancy tracks in order to get the best time. Can be played with friends without chance of collision. Try it for free by downloading Track Mania Nations. United has more terrain types (Dirt, Snow, etc.)
Thomas Was Alone - Puzzle game telling the story of shapes collaborating to reach the end of the levels.
Human Resource Machine - Programming game that gives you algorithms to perform.
Jackbox Party - Party games meant to laugh with friends.

These one I can't confirm they're 100% violence-free, but I have played them enough to be pretty sure:
Seasons After Fall - Really nice experience of controlling seasons to influence the world around you.
Bendy and the Ink Machine - Immersive experience in an artist's old studio that has come back to life (or has it?)
Brothers - A Tale of Two Sons - Walking through the story of two brothers set to save their father. Edit:Violence warning

And these ones harbor some kind of violence, that's for you to judge if it fits your friend's criteria:
Epistory - Typing Chronicles - There are swarms of monsters at time but you need to write words to 'kill' them. No graphical violence. And the game is visually wonderful. It's literally paper unfolding to tell a story.
Psychonauts - Everything is going through the characters' minds. Enemies are 'censors,' people attempting to 'erase' you from memory.
Ibb & Obb - Coop game (2 players local or online) where the only enemies you have to 'kill' are spike balls that disappear when you touch their reflection.

Mario Kart 8

All of these are available on Steam

Splendor
Le Havre
Agricola
Lords of Waterdeep
Bohnanza
Pinball FX3
Jackbox Party Pack
Patchwork
Carcassone
Quell
Loops of Zen
Abalone
Zooloretto
Ticket to Ride
Onirim

I don't think anyone's suggested these yet:

OneShot
Rakuen

Fly'N - There are no enemies to defeat beyond shutting down large machines, but you can be injured yourself

Rocket League, and most sports games.
Also if you stretch the concept of pacifism a bit in the Sonic series you are destroying robots just to free the animals inside of them (most games in the series, not all).

...Hm. Lets see...

Mini Metro
AaAaAA!!! - A Reckless Disregard for Gravity
AaaaaAAaaaAAAaaAAAAaAAAAA!!! for the Awesome
Audiosurf
Clickr (not that I recommend it; it's actually annoying)
Gary's Mod (technically you DON'T have to shoot things, you can build, but...)
Osmos (unless you consider eating other cells violent)
Starseed Pilgrim (...maybe. I haven't gotten very far)
Antichamber
Copoka (not violent, but references violence in conversations[it's set in a distopia, but you're really just flying around picking stuff up])

Myst/Riven/Myst 3/URU (uhh...you're not being violent, though there are references in the background, some execution/torture equipment in the first and second ones that you can poke at that are more story/lore stuff, and some bad ends in 1-3 are...well, bad for you, especially 2 and 3 which have some violence, but nothing really outright violent by your hand unless trapping someone counts)

Obduction
Solar 2 (perhaps violent in a sense, you might want to look and see)
Back to Bed
Bejeweled
Fez
Poker Night at the Inventory
Race the Sun
The Stanley Parable
Mini Thief
Knock-Knock
Treasure Adventure Game (I think, can't remember it too well)

https://www.lorenzostanco.com/lab/steam/u/76561198112076324

Human fall flat
Karakara
Nekopara
Huniepop

Postal 2

Stardew Va- wait no there's the mines. World's Dawn should fill that farming itch but I don't know if there's any combat in it at all but its more social-heavy than Stardew was.
Reccett- no there's the dungeon.
Hexcells! And a lot of puzzle games in general too.
The Room and The Room Two might count.
I never played it much outside of the demo but maybe My Time At Portia?
I never finished Castaway Paradise but if should be only as violent as Animal Crossing ever was.
Do playing with wooden blocks and toy trains count as nonviolent because there's Tracks - The Train Set Game.
Refunct is purely a parkour platformer 'touch the flag' game.
Viridi is a plant growing game.

Otherwise I'm not quite sure off the top of my head.

what about sandbox game like minecraft / ark / space engineer where you can set a peaceful environment if wanted then have fun building your stuff etc
also, racing games / sports game in general.
otherwise there is single player games wich are awesome like visual novels, point and click etc
example : nekopara / to the moon

Your friend doesn't want to play violent games?

Why not go for a stealth game like Thief (the original trilogy or the 2014 one) or Dishonored? They allow you to play completely non-violently, you don't even have to knock anyone out in most missions, if you are up for the challenge ;)

Most hidden object games and point & click adventures also tend to be "violence-free".

Undertale Pacifist Route :p

Seconding Slime Rancher, OneShot and Mini Metro. As for others, how about Celeste, A Hat in Time, or Night in the Woods?

Like a lot of people have said here, puzzle games are a good bet. Brothers is good for that, for example. Seconding that, too.

Tough call, though. Heck, even Portal 2 can get a bit 'violent,' just not with blood.

tropico games - D4: Dark Dreams Don’t Die - broken sword

The ones what i had and remember rn are Rocket league (one of my favs), Meadow, super hexagon, some visual novels, racing games :)

Doki Doki Literature Club?

Tametsi

One of my favourite games has no violence in it at all.

It is called Submerged. A very fun game!

http://store.steampowered.com/app/301860/Submerged/