Thanks Bro!!Yes, he'll be able to use it for multiplayer. Even on his own account, as long as he doesn't delete the account that he got GTA from, from his PS3.
You'll still be able to play all the PS+ games from the IGC as long as you have that PS+ account on your PS3 and the subscription is running but once the subscription on that account runs out, you won't be able to play those games. The Euro account PS+ won't make them work as well. You'll be able to play purchased games forever though.