Wow quite a bit of anti-teacher feeling here! The teaching staff have nothing to do with it. It's the head and governors who make decisions, and no, the teachers do not take time off on day closures! They have a certain number of professional devt hours in a year, and have to record how they're spent.
Having a child miss a day of lessons, when the other pupils are in school is nothing like a day closure when the teacher just makes up the work across the next week or two's lessons.
Sorry if I sound cross, but .... there is so much misrepresentation of what goes on in schools these days.
I suggest mum asks school for the absence pro-forma, fills it in, writes a formal letter to the head asking for a day off, quoting daughter's attendance percentage (it should be on her report), explaining her aunt wants her to be a bridesmaid, that it's the first big family get-together since daughter was little, and sends a copy of the form and letter to the chair of governor and if that doesn't work, a copy of both to the Local Authority