If your friend's being betrayed by another friend, would you tell her ? ?

my best friend has a girlfriend since 2 years ... and i saw his girlfriend making out and getting cozy in a bar with another friend , im sure its not going to last too long but my friend doesnt know anything and always boasts of his girlfriends faithfulness ... should I tell him what I saw ? it is defnitely gonna make them break out or get close to there ... but i feel bad if I didnt ... so its a moral question and kinda delicate ,, what you guys would do ? i know them both very well .