YANBU, OP.
Although, as others have suggested, your fellow student may well be experiencing problems/have issues which you - obviously - know nothing about. However, that's their issue to sort out and discuss with the tutor.
Is she a mature student, also, or younger?
Like you, I was deemed a mature student when I was at uni (I was 22), and the way in which the 18 year olds behaved, their attitudes towards attendance and coursework... well, let's put it this way: I didn't keep in touch with any of them after graduation. In groupwork, someone not pulling their weight affects everyone in that group.
It is difficult, though, because you've essentially just described my daughter - when she's asked to work with a group, it's like torture to her (she has MH issues... which the tutors know about, but her peers don't), and she very often doesn't turn up to meetings (claiming that she didn't know about them) or sobs her way through them because she feels victimised by the others. They, quite understandably, react with confusion, irritation and then anger towards her.
But yes, you have to be brutally honest to your tutor. Because, at the end of the day, this is your grade and your degree and your future. Let the other student worry about their own grade, degree and future. It sounds horrific, but you have to be quite cut-throat about protecting your own future at times - and this is one of those times...