Friend cheating on friend..?

I have a friend, Joanna, and she is datimng, and madly in love with, erik. I just heard from a very reliable source that Erik was cheating on Joanna with his ex...He says he is going to marry Joanna, yet supposedly he is also telling his ex he wants to marry her, too...do i tell my friend that he is cheating on her or do i confront him first?? Or should i just leave it alone...??