Obviously the friend is looking to meet guys and she is using your wife as a wingman.
the problem is that what is your wife doing while the GF is chatting and dancing with guys? Obviously she is chatting and dancing too.
That's it in a nutshell. Are you comfortable with your wife drinking, dancing, flirting and partying all night with guys that want to bang her? And I don't think your wife is just taking a bullet for a friend. She's having a grand little time herself. She's LOVING the attention.
here's a big test: Would she be talking with, accepting drinks from, and dancing with these guys if you were there? Would she be comfortable with you seeing what she's doing and hearing what she's saying?
It's that simple. See if you can go next time. Hang out with them for an hour and see how much she enjoys herself. Ask her after that hour how she could POSSIBLY enjoy these nights to the tune of 6 hours a night every week. Because, you see, when you're not there, it's A LOT more fun. That's the REASON they go to these places without you.
Have her followed. Put a gps in her car. As soon as she parks, have your friend/PI stop by and take some simple cell phone video. I can GUARANTEE her actions will be A LOT different than what she describes to you.
How does she explain away getting slobbered on and groped by strange men all night? How does she even explain these nights now? "By honey. I'm going out to party with strange drunk horny men all night. Don't wait up".
I mean, I ask that question like I can't understand it. Many years ago my wife did the same thing and I can't even COMPREHEND the fact that I stayed home and took it. It's got to be some odd kind of denial. I don't really remember what I went through. What is your thought process as she walks out to door with you giving her permission, essentially, to cheat.
Have her followed. I can GUARANTEE it will be an education.