What do you do when you and your best friend is in love with each other but he has a girlfriend?

You continue to be his best friend and nothing more. As long as he's with another girl, he's not with you. You should make sure to enforce that. You do not want to become "the other woman" in a cheating relationship. Also it would show him not to be a trustworthy person if he is willing to cheat on his girlfriend with you.

If he really is in love with you and not with his girlfriend, he will leave her for you. Until that happens, you continue to support him as the best friend you've always been.