ggirl yes, I did. I went on to do a PhD and am now a university lecturer in music...
So I have known many many people do music degrees.
I would say the most important thing is to find a music degree course which is in tune with the kind of music she likes. If she's really into jazz, then don't go somewhere which has no modules in jazz, yk? And also, look for a course structure which doesn't demand that she should do performance, because some of them do. She'll want to look quite carefully at syllabuses. And she'll want to go to as many open days as possible, just to get a sense of the vibe of each place - there's no substitute for meeting some lecturers and students and thinking "yes, this is my sort of place" or "oh my God, I hate all these people".
As for what people do afterwards... truly, a music degree at a good university can be seen as just another humanities degree. I've known students go on to become lawyers and accountants and bankers. I've known people go into school teaching, both primary and secondary. I've known people go into sales for all sorts of companies. I've known people work in music shops. I've known a lot of people go into arts admin in various guises. Also publishing, either connected to music or not. I've known people go and work for the associated board.
But music can also be a vocational degree, and of course lots of students end up in the music business one way or another, as freelance musicians or salaried orchestral musicians (for the really really talented and lucky ones), as instrumental teachers, as singers in professional choirs.
Getting work experience at music festivals and such would be a very smart move if she's interested in arts admin for the future. But really, I think it's a question of doing a music degree because she can't bear not to know more about how music works