Once a cheater, always a cheater?

[neonsparkles]

One of my best friends asked for my advice on a situation of hers and it got me thinking... so natually I decided to being the topic here!

My best friend and one of our guy friends recently hooked up one drunken night. She tells me that he is now interested in something more (aka a relationship) and she is hesitant about it. The reason? He has a girlfriend.

She wants my advice on what to do... and usually I would just say that if he cheated with her, he will cheat on her.

I've never met the girlfriend (in fact, only 2 of our friends ever have.. but thats another story!) and I haven't hung out with the guy in question much lately due to being in college 3 hrs away for around 9 months of the school year, but he has always been one of those nice guys and I've never known him to cheat before.

So idk, have you ever had a situation where the person didn't cheat again? And if you did like this person, why are you still with your current significant other? Or is it really true that "once a cheater, always a cheater?"