I wrestled with shoving my two cents out, but I’m going to do it anyways.
There’s a difference between being inappropriately dressed to serve a liturgical function and being inappropriately dressed to come into God’s house. It sounds, in my own narrow opinion, that she really didn’t satisfy either.
However, I would rather give her instruction, or make the parish aware of the requirements for serving in some capacity.
I have to confess, I fell into that category once too. I came for confessions before the vigil mass on Saturday evening, and just lost track of time and decided to stay for the mass. I wasn’t dressed nicely-in jeans and a hawaiian-style shirt. A couple minutes into the mass, the sacristan asked me if I could serve as an EMHC, since they were short. I hadn’t planned on attending mass, let alone serving, but the mass was pretty light, and I didn’t see anyone else I knew who was installed to serve, so I reluctantly said yes. I felt more self-conscious that time than I did the first time I served, wearing a full suit and tie. Of course, when we got up there, we realized that the altar servers had forgotten to bring the extra cups, so there were three of us standing there with no cups to distribute, so we had to go sit down again.
The point of this story is, sometimes we don’t anticipate everything life throws at us. Even other people.
Perhaps ask your pastor to do the same thing ours did: turn the AC way up, and keep the Church at a perpetual 60 degrees. That keeps most people fairly well dressed.