Hoo...that's a rough one. I guess it depends on the friendship status of your wife and this other woman. Is this an old friend from wayback? Those can be difficult. My wife had a friend from high school when we were dating (in college) that I just didn't trust. She seemed off somehow, but I couldn't pin it down. I stated my concerns nicely, but at the time, just dating, and friend had tenure, I had to tread lightly. Turned out the friend was a dirtball and my wife hung on with her for a while after I had near-proof of her dirtballitude (My wife had some stuff stolen and the friend was the only one with means, motive, and opportunity to steal it...but no hard evidence), and only later when she was caught red-handed stealing from someone else did she buy in.
Point being, my wife hung with her because, like, BFF, right? I understood that high school friends can and quite often do go in different directions with their lives...especially once they get out of parental influence completely, and it's not fun to say...but they drift apart. She didn't want to let go because she felt friends should be forever.
Yours is a little different in the fact that you're married, so there's a little more commitment there. I think one way to put it is that "it's not that I don't trust you, but I totally, completely, don't trust the two of them. And while I trust that you would never go out and do something that would hurt me, they would. The thing is, no one ever gets up in the morning and says 'hey, I think I'll do X dumb thing today'. So while I trust you, I don't trust those two and fear that they will get you into a situation where something happens that you wouldn't want to happen going into it. Think of it this way...while a bullfighter's wife may trust that he knows what he's doing...she's still probably a wreck during the bullfight, and if given the chance, she'd probably rather he didn't go into the ring in the first place."
What are they planning on doing, what are the odds your wife will end up near drugs or put in a situation where cheating would be easy? The important question is why she wants to go? Is it truly just a great old friend that she wants to do something with? If so, maybe there's some pre-decisions that can be made, if your wife is able to take a stand. If X happens, (Drugs, go to a meat market and the 2 friends are picking out dudes, etc.) you're out, or saying something. If it's just to get away for the weekend, then is it worth it to her to do something that will make you uncomfortable. I wouldn't mind going to the strip bar once in a while, but it's not worth it to make my wife uncomfortable...okay, I wouldn't go there anyway, but it's an example.