Dunno if this thread is still relevant, but I'll assume so.
>>1 I actually had a friend go through a strangely similar occurrence this summer. Likewise, we told him to leave her. He adamantly refused, kept pestering her, she and her group of friends decided to mock his begging for her back though, and she proceeded to tell him she "didn't love him anymore". And now, they're back together. Of course I disapprove of that.
When you enter into a relationship with someone, you're being honest and vulnerable to and with them. In my opinion, the two most important virtues in a relationship of any kind is honesty and respect. When your partner cheats on you, they're taking advantage of and/or destroying both the honesty and respect that was in your relationship.
When you said you didn't want to leave her because of the amount of time and heart you invested in her, it makes sense. But also consider that you've spent that much time on and with her, and she willingly traded it all for a moment with another person.
Have you confronted her about this? How did she respond, if you did? Did she give an excuse and/or apologize? If she gave an excuse at all, I don't think there is no reason to stay with her. There is no excuse for actions like that, only acceptance and apologies. If she made an excuse with you in it ("you weren't here"), then you really need to leave her. Not to mention the fact that she did this with the knowledge that you were not present means she may think she can take advantage of you or manipulate you.
I would not continue to invest any more time or heart with her, because the the cheating may happen again. I know you don't want to see the time you spent with her as wasted, but it isn't. That time wasn't wasted, it was kept and logged in both of your books. You guys had a great time, but it's time to find someone else with more respect for your heart and your time.
>>12's advisor's advice is very good.
But enough of my opinion. The only advice I gave to my friend when this happened to him was this: you're a grown and mature adult now, you can make you're own choices. You know what we (your friends) think and our opinions of your situation, but this is your life and your choice. Just know that whatever you decide, to pursue or leave, that you must stay firm in your choice and what you believe even through the toughest of consequences.
Good luck, mate.