Thank you all for trying to inject seeds of doubt in me, by the way, I repeat, I am sure of my own situation, I already cleared it months ago (again, not going to write anything more to explain why I'm sure of my situation), and the few facts I wrote are to explain the guy why it's not said that a girl going on holiday with friends will cheat. The point is, GFs can and will flirt anyway, but they aren't going to give away their uterus like that.
The problem here is that you don't get to know the aspects of real life that give you the 99% of information about the situation: so you don't know the guy and girl's looks and personality and frame control. And everything is biased by people talking by extremes, so it's gonna be an endless debate on trying to explain to people that girls DON'T cheat all the time, and that it's a sentence made for hopeless afcs to make them be less of a servant to their girlfriends.
The only guy here who can have a clue on the situation is OP, and what we can do best to help him is to make him move his energies away from the worries of his relationship and channel them to more constructive purposes. The solution is not "go and have sex with other girls" or "be always anxious and be sure that your gf will cheat", but it's "don't give a damn about what you can not control, and keep going". And I'm sorry for you, fearmongers, but from what the guy wrote, I think it's gonna be hard that his gf will even talk to any other guy.