Like why would you want to fuck over this dude? I get that everybody wants to be an alpha male and shit, but getting cheated on isnt something so simple.
If you go do shit with your ex, the dude is going to get fucked over. If you really wanna smash, do it and immediately tell her boyfriend. But it would be much better if you just block her and tell her boyfriend that she was trying to cheat.
Odds are that she has probably cheated before and might have some STD or some shit.
Dont be a fuckboy cheater.
OR imagine you being in the dudes position and some asshole sleeps with your GF and you dont find out about it for months. She could have gotten an STD or some shit and given it to you.