Actually, she screwed the other guy AFTER that dude was like *no no thanks I'm over you*. And they weren't dating, so how can he call her on anything? He was asking out of his own interest, not discovering evidence or proving she was sneaking or lying.
Ok, so here's my penny's worth. It wasn't a good idea to stay FWB with him after him confessing those feelings and stuff. That breaks up what I thought was the major rule of FWB-i.e. not being a relationship, and not having relationship-type feelings except from lust.
Maybe it's just a guy's way of dealing with not having who they want. Maybe it was this particular guy's way of dealing with it. Just flip a V bird in the air in tribute of him and move on.
p.s. Never grovel or beg a guy for forgiveness. It is a sign of weakness. Especially if it wasn't your fault!!!! And DEFINITELY DON'T text guys any words of grovelling-ness.