Competition win

Ask him again that he/she confirms the game.
If 7 days have passed contact support and they open a can of whoppass on him/her.

He has 7 days to mark it as received, if he doesn't do that in 7 days contact support with proof and they gonna mark it for you.

wait for some more days, then contact support

Wait 7 days. If they still haven't marked it contact support. I'd probably include a screen shot of you sending it (email, steam gift) and wait for their response. Might take a while for them to respond unfortunately.

You contact them and ask them if they received it/to activate it and record the conversation/take screenshots,then make a ticket with that to support. If he avoids you,wait 7 days,then send proof he received your message to support.