I am 5ft 5in and weigh 13 stone OP. I am fit and strong and healthy. I can kayak for an hour, I do around 12k steps a day without breaking into a sweat, I work on my allotment at least once a week and we have 2 ponies, 1 of which I have just broken in by myself, including getting on (which is a good sign of fitness imo) without any help.
At 5ft 9 she is probably a size 12/14. Shops like H and M, Topshop, New Look should have appropriate clothes for her. Dd has been wearing size 8s since she was 12/13 when most of the children's ranges finished. She's now 15 and wears a lot of Jack Willis stuff, and likes shopping in Hollister etc. Clothes shouldn't be a problem for your dd. My dd is very conservative, doesn't wear tight or revealing clothes and I don't know why you can't find appropriate clothing.
With regards to her health and fitness I would listen to her sports coaches over people who haven't seen her on mn, where anyone over a size 8 is classed as a bit porky. Don't damage her self esteem or her amazing confidence by saying she shouldn't do dance because she's too tall, or she should go on a diet. Feed her healthy food, not diet food, healthy food. Has her periods started yet? Dd filled out a bit before she started then dropped down again once she was in a regular cycle.
Don't compare her to anyone else. She sounds amazing. Don't knock her confidence, she will need it for the teenage years. Support her in whatever she wants to do sports wise and take her shopping for new clothes to trendy teenage approved places. At her height it shouldn't be difficult, teenagers are taller these days and lots of places do a taller option now. 5ft 9in is tall but not massively so in adult sizes.