Hey, shit happens. So as long as you ditch him now you can keep your head high. There's no %age in going out of your way to tell her unless you happen to know her or have mutual friends. She won't thank you for it.
If you do have mutual friends, the way to handle it is to drop something to them like "Yeah, I was making out with XXX the other night. Until I found out he was seeing someone. I'm not into that, so he was right outta there. Feel bad for her tho, wonder if he'll fess up?".
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh