I've just finished A level Dance (waiting on results in August!!) so can definitely give you my opinion on this.
It depends on what exam board you're sitting but I did AQA and it was 55% written and 45% practical although we spent much more time doing practical work than theory. If she definitely wants to take biology then she'll no doubt be familiar with a lot of the physiology stuff that comes up in theory which will be handy. The practical work is generally of a contemporary style but the people in my class came from loads of different backgrounds (gymnastics, ballet, ballroom and latin, hip hop, jazz, musical theatre, tap etc) so it really doesn't matter what genre you're most experienced in. I personally loved A level dance and couldn't recommend it more, although it is a heck of a lot of hard work and you have to be prepared to put in a lot of your spare time rehearsing and choreographing. Some people in my class in year 12 only took Dance as far as AS and are going on to do things like Psychology and Languages at University and from what I know, an AS in something more practical like Dance was very helpful to them when applying for Uni.
If you have any other questions, I'm happy to give advice!!