Should I tell the husband his wife is a CHEATER?

So, found out that my fiance had an encounter with a married woman not once but twice, this was of course before we me and got together, I have actually talked to this girl online, never met her in person, but wondering should I tell her husband, I have all the proof he would need, I could just foward the IMs and the emails and he could see for himself. If someone cheated on me I would want someone to tell me, what should I do?