My two cents (taking into consideration that I'm a dude answering your comments)

First, I presumed that the trip with the gf and the kids was sold to the mother (and possibly the husband back at home?) as a chance for mom to see her grandkids for a couple of days and a chance for me and mom to catch up like old times. From mom's letter to Prudie, it seemed that was her thoughts on the daughters visit as well.

The daughter cooks it up with her gf in advance and leaves the mom in the dark. Maybe because she knew the mom wouldn't approve of the two women going out alone, bar hopping and staying out until early the next morning, and would maker her objections known? From the wording of the LW, it certainly sounds that way.

Regarding the argument that young mom's deserve the right to go out once in a while.... i agree, but they need to be upfront and honest about their intentions. This trip has deceit written all over it. Maybe the mom wasn't drunk in front of her kids. But maybe she came home in the wee hours and was loud and stumbled into walls or knocked over furniture on her way to bed and woke them up? Maybe she and her gf stayed up a couple more hours giggling about the cute guys hitting on them and buying them drinks all night and woke the LW up? Maybe the LW had to make excuses to her grandkids why mommy needs to sleep in until after noon every day and can't be up visiting and playing with them at the park?

Too many factors that aren't answered here. But something just doesn't sound right about this situation. And the LW has every right to let her daughter know she isn't going to be taken advantage of again.