The point is that you have no guarantee you're going to sleep with her. I know it sounds like I'm suggesting the trip is, or should be a financial exchange for sex. That's not it. But men have to protect themselves from women who want to use us, and one of the best ways to determine if you are being used is whether or not she is sleeping with you. Sure, you can still be used if she's sleeping with you, but a $1k USD trip isn't really a big deal assuming she is having sex with you.... we aren't talking about her expecting expensive gifts or lavish dinners etc.
Just imagine if you go on that trip and as soon as the wheels of the plane touch ground, all the kino stops and she friend-zones you and tries to demand you sleep on the couch. How enjoyable would that week be? Would you still be happy about spending that $1k and a week of your time on her? And imagine she and her friend having a laugh at your expense over your thinking this trip was going to be something it wasn't and her "putting you in your place."
Don't let your anger over the friend blind you about this. Banging her at home would still accomplish that, if that is important to you.