Meeting a girl on dating app just for sex?

[Kevin]

My friend (yes my friend and not me) had recently downloaded an app called hot or not. He doesn't get very many girls and is still a virgin (20 years old).
He got matched with a 19 year old girl on the app and they had swapped snapchats and she immediately started sending him nudes like back to back to back. I thought it was a little random and odd but maybe she was just a slutty girl. Then he tells me they're gonna hang out next week and have sex. (So she says) I wanted to make sure it wasn't something odd going on so I looked her up on Facebook and it says he has been in a relationship with her boyfriend for he past 3 weeks. The literally became boyfriend and girlfriend 20 days ago and her profile picture is with him aswell. Idk what the hell Is going on and maybe she is just a tramp but I don't want anything bad to happen to my friend. Something just feels odd about this whole situation and maybe someone could give me some input and if this sounds safe or not.

[hijikatuski]

Honestly for some people sex just isn't a big deal. I personally hooked up with a guy who at the time was in a relationship, granted I wasn't aware of it at the time, but he and his girlfriend had an open relationship and they allowed each other to sleep around. It could be the same situation although I have to admit this girl seems extremely fishy, the fact that he does not actually know her is worrying, in my situation I knew that the person I was talking to was actually one of my peers. I think he should be extremely cautious with this girl because although she and her boyfriend could be very open with each other it is more likely he is getting cat-fished.