How do you deal with other guys your plates/fwbs are having ?

Im not reading much about it here. Surely not everyone can except your plates to have only you as their only partners while you fuck around ?

I have a fwb since roughly a year. She went to holidays with another dude. "Don't worry we don't even share a room". I'm not naive and I didn't expect her not to fuck him. What gets to me is the blatant lying. She's just back and I'm pretty turned off by that whole thing. The dude also posted some pics on fb of them holdings hands and all.

Lying is much more common here in Asia to save face and all and it still pissed me off. Also sucks to loose her as a fuckbuddy. Great rack and very open for everything. I don't even know how to demote her, honestly just thinking of kickin her out of rotation.

What would you do ? How do u deal with those situations normally ?