Gifter send key, winner activate on different account, mark as not received.

I belive so, email or chat history some kind of proof that the key was sent.

If you have proof as a gifter that you have sent key you can make a support ticket if the key is not marked redeem

Redeeming a key from a giveaway on a second account that did not win the game is punishable by ban for both accounts in question i believe

what if the winner claimed the key is not working and claimed a new key from gifter?

good point, that's pretty fucked up then. I wonder how you will defend yourself if the key is from a bundle even though you store perfectly unused keys in a file.

If it's a retail key then I can provide receipt.

You can check your redeemed or purchase history from your profile in the Steam Client by clicking on Account Details.

Whoever would do such thing deserves a place in hell

I guess the only way would be if the gifter would proof that he aquired the key to Steam support and then have Steam support check who activated the key and when.
Steam support will then probably remove the game from the "false winners" account, but afaik, they will not give a new key or make the used key usable again.
But with this information from Steam support, the gifter could proof to SG support that he originally delivered a working key and then the winner and the other account will be banned.

So your friend is a scammer. Good to know :D

Send keys via steam chat, especially on $0 cv giveaways.

Also, these sorts of questions tend to sound like "hey, I thought up a scam, anyone see a problem with it?"

Good story

It's easy to do but, also, it's quite pointless (you damage the gifter without benefitting yourself, as you did actually win a free game)... It's not even a real "scam" as we usually intend that...

I've always wondered the same thing myself when sending keys but you'd have proof that you got it from GMG or a bundle and proof you sent it in email or chat.

This is why I give giftable copies of games and not keys. There's a section in your account dedicated to keeping track of transactions and trades.