Throw him a party of some type. Get his gf in on it. Order pizza and drinks and stuff like that. Then get him a gift card for something he enjoys doing. I figure since you are going to spend 50 dollars on a male friend, you are probably good friends, and know what he likes to do.
If he wants an MP3 player, get him a cheap one, or go in with someone to get him a nice one.
Does he need clothes? Or cologne? Shoes? May'be some tools? A beanbag? There are lots and lots of things you can purchase with $50.
Perhaps something for his vehicle, if he has one.