How do you get your friend to tell his girlfriend he has been cheating on her?

Good luck - that would be hard task to do however, you could just simply try to talk to him and tell him it is not right what he is doing and not fair to his girlfriend. You have to know this could cause a riff in your relationship. You could also send an anonymous letter to his girlfriend as to not be involved and this way she will find out and you will be able to keep your friendship if you wish too.