Drama classes, preferably ones offering grade level exams as that gives some indication of the curriculum and the standard of teaching and offers qualifications.
(If she's looking at musical theatre, she needs good singing and dancing classes, too, and not just the commercial competition type, but if she's had no dance training at 14, she's going to find it very tough.)
At the same time, look at the NODA website, find a couple of local amateur societies, go watch their stuff to get a feel for them and have her start auditioning. The standard can be incredibly high, depending on the society, and it's the best way to really decide if she loves it.
If she's serious, she needs more 'on her CV' than school plays and school qualifications - acting school places are very competitive and the better rated drama/acting BA's are also academically tough, needing high grades out of sixth form.
It also lets you be supportive while testing whether she's really inclined to the reality and the work it needs but there are absolutely loads of careers in the performing arts and surrounding them.
And, even if she doesn't go for it, there are tons of transferable skills.