I remembered meeting up with a friend of mine who initially didn't have a gf, but after he and i stopped meeting up due to work related reasons, he found himself a gf.  We were still communicating with each other and he eventually suggested that we should continue meeting up even though he has a gf. His reasoning was that we're young once, we're both very compatible in certain ways and should give in to our desires simply for that.  I was very tempted, I mean, I could literally drive over to him if I  wanted.  But all the time he and I talked about this, I couldn't bring myself to just go for it.  I kept imagining myself in his gf's shoes and I didn't want something like this happening to me.  So after some time thinking about it, I told the guy. He said he understood.  We still talk but not as much.  I would say your friend isn't to blame for her ex's mistake, but she did hook up with him knowing full well that he had a gf.  She may not have tempted him into cheating but it does take 2 people to make it happen.   I'm not saying it's her fault, I'm just saying she could've stopped it from happening.  Just my 2 cents. :)

yes, it is her responsibility.

that's fucked up.

and karma will get her.

It is both their fault.

The guy for being a disgusting cheater and a liar.

The girl for going along with whatever he wanted, knowing that he already had a girlfriend.

Sorry to be so blatant, but it's the truth.

Both parties are to blame, but I'd shift the degree of blame based on who initiated the cheating relationship. If the single person is predatory and continually trying to break up the couple, I'd put the cheater and the single person on equal footing. If the cheater initiates the cheating relationship, I'd put more blame on the cheater than the single person.

I'd also put more blame on the single person is actually breaking up a "relationship", and by that, I mean where the two people actually love each other. If they don't love each other, but are still together because they don't want to go through the hassle of divorce, they have been together so long that they are "comfortable" with their routine, or are just together because they depend on each other for rent, I'd give the single person even less blame.

Cheating, to me, is a lot less serious if the parties are still together, but are mutually cheating on each other or don't love each other at all.

I am sorry but yes. It is your responsibility. Reject the cheater, at least that is what I have done in the past and will continue to do in the future. 

Its both parties fault

One may feel guilty or more innocent then the other but if it causes a problem, you both are to blame

Well, without girls willing to help cheaters, there wouldn't be any cheating. I know girls who continue to do things with guys they know have girlfriends. It's one thing if they didn't previously know, but as soon as they find that out, it should be a no brainer to run far away.

 I don't know how they could knowingly and willingly help a guy cheat. Despite the feelings you have for that guy -- he's obviously scum if he's willing to use you to cheat. He obviously doesn't really truly care about either girl involved.

I'd never help a guy cheat, nor would I cheat. 'Nuff said.

Also, if you help someone cheat, it wouldn't surprise me if they would feel guilty. It's their fault just as much as it is his, especially when they know what they were getting into.

I agree to a certain extent. I've been recently messing around with this guy (The Cheater) who he's been with his girlfriend for four years, on and off. I secretly had a crush on him Senior year but didn't quite introduce ourselves until three years later after I received a random email from him. At this stage in my life, I'm living the single and free life. I had dealt with the most traumatizing break up with my Ex Boyfriend of two years and ten months two years ago and is now looking for nothing more than casual fun dating/hook ups. Just so coincidentally, that's when I received an email from him. I did not know anything about his girlfriend until the day we hung out (and that was after 3months of exchanging emails) and it didn't take long till the third day we hung out was when messed around. Of course like every woman who has never been in this situation, felt remorsefully guilty but at the same time, it wasn't my say to tell him what to do with his life. So down the road, after bottling my guilty conscious in a bottle, I had to tell him that what he/we're doing is wrong. That cheating on his girlfriend is wrong and should be stopped. But he insisted.

In his theory, he feels that I shouldn't worry because it was his idea in the first place to do this. That until he is tied down (as in married), he can do whatever he wants, date whoever he wants, and cheat if he wants to. How he's done far more awful things in life and this is the least of his concerns. That he's young and again, can do whatever he wants so he doesn't see why I should be at all guilty. So with that being said, (also after trying  a week or two with the "ignore" button in hopes his light bulb worked) I gave up on the matter and decided to just go with it. As for me (the other woman), I'm fine with it. I'm young too, but single and free of that relationship drama. I also know I won't ever get attached with this guy because everything about him is total opposite of what I seek in a Boyfriend. So I've come to terms that, until he finally realizes what he wants, or my future boyfriend came into my life, he'll continue to cheat on his girlfriend and I will continue to be that other woman.

Anyways, we're just as good as friends as we are hooking up. But I know for sure that when that SO does come into my life (and that's when I do decide to end this fun), this would not be tolerable in my book. I have been the one to completely devote myself to my SO and end up with the ugly stick (so to speak). I see things simply in black and white. If you're not completely happy with the person you're with, then why not be honest with yourself and to the person and end the relationship rather then be in it feeling miserable? But hey, in this situation that's non of my problem! You can't help change a person unless they're willing to change for themselves. Sometimes it takes something like this for one person to realize what they really want in life. We all make mistakes and we all learn from them.