there's no easy answer there, but if he cheats on her, odds are good he's gonna cheat on you. life is too short to waste time on liars and cheaters.

To piggyback (and agree) with montypython103, if he truly found something in you HE should break up with her; not have her break up with him and leave you as the 2nd choice. sorry

Does he want to be with you? I've been in this scenario with my ex, I felt bad that she was sleeping with me and calling him baby on the phone while cuddling with me. Sometimes ignorance is bliss; as bad as that sounds. Make sure you know where his heart is before you make a move

Warn her. If she keeps going, you did your part.

Warn her. it may hurt but in the end it will be easier for her to let go and heal

Sorry to hear about your situation, he sounds like a bit of an arsehole to me. So unfair you're made to feel second best, you don't deserve that!
Totally not your fault, this is all on him.
I think you're wise to want to warn his girlfriend. If he's been seeing you and leading you a merry dance while going out with her, there's a strong chance he'll do it again with another girl.
If I had my way, two timing bastards like him would be castrated!
Sorry for that last remark being a bit much, but infidelity is one of my pet hates.

Chin up babe, you'll find someone else who'll love you and respect you and treat you how you should be treated.

Always tell her. Be it because you want him (sounds like he doesn't deserve you but...) or you're just having morals.
Girls need to stick together & have each others back. Being in the dark is the worst.

i think you are right you need to tell her,like that at least you did the right thing and know that for yourself.and that guy does not deserve you or her!its not ok to play with people and someone who did that once will do it again!you are to good for that never forget that!

That's a bit of a landmine

Let her know. It will be possibly the worst and most uncomfortable conversation you'll ever have, but you'll both end up better off because of it. She'll be angry with you in the short term, but she'll thank you in the long term.

Totally agree with Sif and vertizontal, she'll thank you in the long run.
You can do far better than a guy who wants to have his cake and eat it.
Never let anyone make you feel second best.

You should tell her but this will open up alot of hurt on all sides.. good luck ..Ive been there..