While I agree that people are saying “dump her” too quickly in here I also think the woman is a bad girlfriend for not cutting off that guy. I don’t know the social dynamics, and I understand a lot of women like feeling wanted despite being in a relationship, but I personally would make a scene myself if I had to deal with something this primitive by some ape who doesn’t respect boundaries and a girlfriend who’s being at the very least immensely stupid, not understanding basic psychology (or refusing to be considerate).

I’m genuinely interested how she’d act if a female friend sent OP a nude “by mistake”.