Two things. Just because you are married only 6 months does not mean she is not cheating. A young couple we knew socially were engaged. The guy was cheating on her before the engagement, through the engagement and after they married. The only thing was that his old excuse of working late to get them more money did not work when he wife saw the paychecks and put 2 and 2 together. She divorced him.
The second problem I see is that two girls drinking heavily is a recipe for sex with strangers. I have had sex with a few married women during business trips after a few drinks at the bar of a hosted party by a client. After having my ex fiancée and then a girlfriend cheat on me, I will never trust anyone, male or female with a guy after they have been drinking which lowers their inhibitions especially knowing that you will never find out.
I am a different kind of guy. When I went on vacation I not only took my wife but also our girlfriend.
I have been burnt by an ex fiancée who I never once thought would cheat. She never even looked at other guys, or once said anything about another man she saw as we walked around the mall. She blindsided me because I was not looking for clues and made it easy on her.
It could be just a weekend away but why does it have to be a weekend without you? She could still drink if you go with her and party too. The only thing she could not do if you were there is flirt with guys, get drunk and have sex with them. Where there is alcohol and men, no woman is safe, no matter what her initial intentions were. I have been on countless business trips and had drinks with my co-workers and other business men adn women. After a few drinks, the flirting starts. Far from home and no risk of being caught.
You can always look at it like this. If she has sex with a stranger that she never will see again, if you do not know that, it did not happen in your world. The danger is if she is going to meet a boyfriend. I dated a married women who used to meet her boyfriend at business seminars, conventions, trips with the girlfriends, etc. I asked her how she got away with it and she said that her girlfriend and even her secretary covered for her. I used to cover for the President of one company I worked for. I would tell his wife what he told me to tell her. Have a healthy does of mistrust and it may save your marriage. I am old enough to see what married women do when drunk. One tried to perform oral on me, another was tongue kissing me while grabbing my penis through my pants and trying to jerk me off. On the dance floor they would press against my crotch. Drinking and women do not make for a safe trip.