If you

Well he may not be cheating, he may have had the app (or been on the site) when he was single but not use it anymore now that he has someone. Some people just stop using it over time. Know what I mean? Wouldn't want to start a fight if there's nothing to fight about. But if he's active and seeking out women, you go right ahead. You could always like make a profile (fake if he knows you) and send him a message. Catch him red handed

Do u know them well?? 

Depends on how well I knew his girlfriend

That's how I found out, a complete stranger to me but a friend to him. 

Completely depends on how well you know the people involved. If you don't know them well then for all you know they could have an open relationship. If it's a friend I'd say something. Whatever the situation if you choose to get involved be aware that a lot of the time the messenger ends up being the target of all the anger from both sides.

Been there done that...at the end everyone was mad at me!The messenger always gets shot!Would I do it again in similar situation?! Maybe...because I would expect my friends to be honest and do the same thing

I'd have no problem calling out a cheater. And if it's not recent, it should've been removed.