Her relationships are her problem, as are the consequences. What if you stick your neck out and she continues to violate the kids’ trust. You don’t want to lose the relationship you have.
It is also somewhat juvenile to take this approach. This is her mother and she is a big girl. Mom needs to work this out. Once you are gone from the home you won’t be there to help. Distance can help.
College kids are vulnerable and miss home. Mom can send emails, letters, care packages, etc.
I think it is very normal for kids to feel the violation and act out. If you step in here you teach W you’ll be her fixer and you prevent her from reaching down and figuring out a way to get past this.
