In her defence, she's young, her friends are sluts, she doesn't want to be the one ruining 'fun' on the trip, and also thought she could live her life and that her boyfriend was a bit of a pushover and she could control him (reason she's with him). She probably isn't as much as a slut as her friends or even one but got swept up in their sluttiness.
The guy, what a loyal guy, could have apparently cheated on her and didn't. So she opens it up and then gets jealous he does it and does it first! Then she's pissed it's a not a one night stand thing it's regular. Yeh so why would the guy go and fuck some drunk shanky slut which would take effort anyway when he obviously has something better sitting there. She made a bad decision and fucked up, the guy will probably not go back to her hope not.
