How good a friend is he? lol

Probably, $50,000 is the right answer, but what if your friend refuses to take the money? Was the $100 a birthday gift or just money to gamble with? If the money was indeed a gift, there's no moral obligation to share the winnings. But if he's your friend, then sharing some of your winnings is certainly a way "give back." I think the appropriate response is to take your friend to dinner, tell him you won, and ask him how much of it he'd like to have. It could be a way to strengthen your friendship.