I dated a girl who used to go out after work like every night, and most of the people with her were guys. It used to bother me, but then I started going out myself when she did. Even if I wasn't doing anything I'd ignore her and act like I was busy. Not after long she stopped going out after work.
So next time she go's out with the girls, say ''cool, I'm gonna meet up with the guys at xxxbar'. Don't be surprised if she shows up.
But why should he be dependent on what she does though? That creates dependence issues.
My wife goes out once in a while. Do I worry? No. Why? Because if someone else is dumb enough to fall for a "used doll" they deserve to get that.
What I will never do is ask myself why. Did I fuck her right? Did I treat her well? Who the fuck cares! Life is literally TOO SHORT to have to worry about another human being that much. What you really want to do NOW is stop worrying about shit like that. If you find out that she cheated on you, break it off and move on immediately. If she didn't, but she still wants to go out once in a while WITHOUT YOU? Bullshit, there's the door. Have a nice life.
Kids aside, no one should be that important in your life.
My grandfather once told my grandmother, who was suffering from a bout of depression, to "walk it off". Eventually she did get out of the mental funk, but it wasn't because of what he said.