I agree with others above - is it that she finds basic school maths a bit boring, or that she doesn’t really love it? I do think you have to enjoy maths if you’re going to try to do loads at a level, especially further maths. And with university physics depts, it’s entirely about what the cohort generally has. There are loads of good physics degrees where most of them don’t have FM a level, and the course will be paced accordingly. It’s all about doing it at the right pace.
And as others have said, have a think about engineering. It may well be that side of physics she likes, and while some engineering courses like FM too, it’s a less fundamentally maths based thing than pure physics. There are loads of videos on various engineering degrees on Imperial college website (was looking at them with my son recently).
Or, look through the biological sciences natsci options. There are different maths courses for people who aren’t taking physics / chemistry. Can’t remember exactly how it works. I wish I’d done those ones! Would have been v useful. But I was expecting to do full on chemistry which needed the physical sciences maths.
Good luck to her - it’s so hard thinking about what you want to do for your degree when you’re only 15. But the more you can look into options, maybe go to uni fairs at schools etc, the more she can build up a mental image of the possibilities.