I am the other womanWatch
I really like him and he makes me feel really good when I’m around him.
Not that you come out of this well. Do you also feel good that you are basically encouraging someone's poor behaviour?
Out of the four or five guys that told her this, none of them left their girlfriends, but they were all willing to come have sex with her behind the girlfriends back.
If he did leave her, have you thought about the fact that he will probably do the same thing to you? It sounds like he’s a player and doesn’t take relationships seriously and wants options.
My advice is to cut him off (even though it sucks) but it could save you from heartbreak later on. If it were me, I would tell the girl because she deserves to know what he’s doing. It’s sad to think she’s out there thinking her boyfriend is faithful and I hate the whole “if she only knew thing” because it really is hurtful. You’d probably want the other girl to tell you if the situation was reversed.