Good Co-Op Steam Games

Payday?

If he loves HL2, you can try Synergy

IF U LIKE L4D THEN ULL LIKE BORDERLANDS AND BORDERLANDS 2. AWESOME GAME!
I HAVE SAME PROBLEM WITH MY HUSBAND, WE PLAY ON LAN MODE. BORDERLANDS AT LEAST 2 - IS SUPPORTING CONTROLLER. I DON'T REMEMBER WHAT ABOUT BORDERLANDS 1. BUT U CAN TRY, A LOT OF FUN, MANY QUEST, NICE SHOOTER WITH ELEMENTS OF RPG LVLING AND HUNTING FOR BETTER GEAR.

TRY BORDERLANDS!
PS. IM NOT SHOUTING, I'M AT WORK USING CAPS LOCK ALL THE TIME :) FORGIVE ME.

Portal 2 is a good coop puzzle game.

dota 2 :-)

portal2,orcs must die2,l4d2,killing floor

Saints row the third is extremely fun coop.

Dungeon Defenders...?

Portal 2 has thousands of Workshop coop maps along with the ones in the vanilla game.
Also Castle crashers, mindless fun, but loads of fun. Rock of ages I found annoying, but might be fun if you're good at it. Sanctum 2 is also fun tower defence/fps.
"Monaco" is cool, gets tense, yet great humor, you'll probably need 3 people later on when it gets tougher.
Orcs must die 2 - made for 2 player coop. Very fun.

All with controller support.

Sacred 2 Gold might be worth a look - you'll have to use Tunngle, though, to play in co-op.

Arma can be played co op missions only if you want, although i would then go for the second one which i think you can get for free and the arrowhead expansion, both for price reasons and for the greater amount of co op missions created by users as compared to the newest title. The latest one is probably not worth it if you do only co op.

Resident Evil Revelations

killing floor, trine 1+2, alien swarm

w8 for division

I recommend Magicka, Castle crashers, Rayman Origins and Resident Evil 5, aditionally you can search for the best coop games (based on metascore) on Steam: Best co-op games

Forced is a great play with friends

Dark Souls.

Try Orcs Must Die! 2 and Trine 2. For some competitive play between you two, try Civilization V and Frozen Synapse (Highly Recommended).

Magicka. Oh my God have I had fun with that game...

Splinter Cell games, tho seems that all except for the last one you need to use tunngle/hamachi to be able to co-op since they closed all online co-op support.

Borderlands; Sacred 2; Portal 2; Saints Row 3/4; Hammerwatch...

Not a coop game, but Sonic & All-Stars Racing Transformed has great (4 player local split screen) multiplayer support (I actually prefer it to mario kart)

Orcs must die 2 -
Operation Flash Point Dragon Rising -
Portal 2 -
Killing Floor -
Payday 1 & 2 -
Left 4 Dead 2 -
Saints Row The Third -
Far Cry 3 -
Magicka -
Sanctum 1 & 2 -
Civilization V -
Terraria -
StarBound -
Trine 2 -
Painkiller -
Torchlight II

Trine 2, it's a lovely co-op game and you can have 1-3 players. It is the most fun co-op game I have played and flawless controller support (customisable controls for everything). So this is number 1 on my recommendations list.

Portal 2 co-op is also pretty fun, a tad more difficult though. Orcs must die 2 is also pretty fun, but can get redundant. Terarria can also be fun, if you're into those kind of games and old-school pixel graphics don't bother you.

Did you ever try Payday: The Heist, Payday 2, or Killing Floor?

Having just played a bit of Risk Of Rain, i can say its probably a hoot in coop mode