I took Maths in year 12 and further maths in year 13. I personally liked it best this way because I wasn’t that confident with a lot of maths topics compared to others in my maths class, and the straight maths year gave me time to catch up.
For example, AS maths starts with expanding brackets and AS further maths starts with complex numbers. When I started year 12, I genuinely wasn’t confident enough with coordinate geometry to understand the initial further maths topics, and probably would have quit pretty soon if we had to do it that way.
Like someone else mentioned, it also means the topics will be taught in a sensible order, without having to jump around. It also means she will definitely be in a maths class with other further maths students at the same level.
On the other hand, some people would argue that taking the maths exam a year early is risky. If she got a bad grade, it would disadvantage her when applying to universities, even if she’s planning to resit in year 13. Getting a good grade means she has one A level out of the way though, which will give her more confidence.