I teach biology at A-level, and it is a big step up from the GCSE- at GCSE I would say for many it's the "easiest" science, and so students find it enjoyable- but the jump to A-level is wider (in my opinion) than chemistry and physics. Chemistry and physics usually start with content that's a bit more familiar and then build on it, biology is often more "in at the deep end".
There's also a lot of content to learn, especially if she's not doing another science. Biology and chemistry really support each other at A-level. She's also not necessarily keeping science at uni open as an option by only doing one science. Really you need to figure out if she might want to do sciences in the future- if she does, doing only one science will drastically cut down options.
I'm not saying that she won't do well, with 9s, but it may not be the best 3rd choice.
I'd consider English, economics and maths as a possible combination too- a lot of unis will want maths if she wants to take the economics further anyway.
You can do language or literature as A-levels. You can also do combined lang and lit - I think it's slightly less respected by unis if she wanted to go down an English route, though. Teachers will be able to advise about English language A-level, I think it's quite linguistics based and different to the GCSE, but it's worth speaking to her teachers.
In terms of what she'll enjoy, what does she enjoy now? Students who enjoy sciences and maths at GCSE tend to enjoy them at A-level. Students who like English/humanities at GCSE tend to like more essay based subjects at A-level. Maths especially I do think you have to really love it to want to take it past GCSE, it's not enough to just be good at it.
Not sure how much this helps, but I'd get her to consider broad uni options and work backwards from there!