I (18M) accidentally met a girl I hooked up withs’ boyfriend and she’s lying to him. Should I tell him?

I’ve (18M) been in college a couple months. I hooked up with a girl I met like three weeks ago. She asked for my socials from a friend at a hangout and then we ended up sleeping with each other the next day after grabbing food. I don’t really talk to her much after that besides some awkward waves while passing each other

What I know about the girl from what she told me is that she likes to party a lot and partied everyday of summer. I also heard from a friend that he hooked up with her during summer.

A week and a half later she gets a boyfriend. I hear from her friend she really likes him and is really happy. I think good for her and was happy she found someone she liked because she also told me that men only wanted her for sex. (Which was pretty disheartening).

I think nothing of it and occasionally see them post on instagram together in matching outfits. A good amount of time passes and I’m hanging with a good friend of mine. We’re walking around the campus when he sees one of his friends and starts talking to him. In my head he looks familiar but I couldn’t tell where I’ve seen him from.

My friend asks him about his girlfriend. He says “Oh [girls name] yea it’s been going pretty good”. He then goes on to detail their relationship. He tells us she took his virginity and a variety of other relationship things I wasn’t really worried about. Then he tells us that she doesn’t like to go out to parties and likes to stay inside. My friend asks him what her body count is and he tells us only two and that they were together for a bit before they did anything.

So now I’m side eyeing my friend the whole time as he goes on to tell us a bit more about her. After he leaves I tell my friend everything. He asked me why I didn’t say anything and I told him it wasn’t really my place to tell him. He told me as a man I should’ve told him that his girlfriend was lying to him and that it wasn’t worse because it was to his face.

I asked him why couldn’t he tell him and he told me it wasn’t his place because it wasn’t his business. I told him I felt the same and he disagreed saying I directly knew that she was lying to him and didn’t say anything.

Should I tell him? I really don’t want to because I don’t think it’s my business and idk why she lied to him or what she’s thinking but everyone I ask told me I should’ve told him and are advising me to tell him. I barely know him and don’t want to just start a conversation about his girl lying to him idk I just don’t think it’s a good idea.

What do you think about it? Would you have told him? How should I tell him if I do?