Does she like competing? Or does she solely like training? If she doesn’t get as much out of competing as she does from trying to improve her ability in training, maybe that is why? And/or competition has a lot of room for thinking other people might think badly of one, maybe this potential drawback of competing outweighs the benefits.
I play a competitive sport and while I really enjoy training for it, I have very mixed feelings about actually competing. When competing or contemplating competing, I feel very worried about letting other people down and about messing things up or failing or not making it on a team (it’s devastating when it does happen and throws me off for months). It worries me to a degree that I pretty much hate competing. (When the game is over and I’m with my teammates who are supportive of each other win or lose, I like it, but beforehand I hate everything about it and worry for weeks/months and wonder why I do it at all.) When training I am enjoying improving my skills, company of friends and the exhilaration of the movement itself. For me, the solution has been to step back from competition occasionally and just enjoy doing a thing I like for its own sake and being involved in the community.
Edit: Also, regarding how ADHD people train for sports. I can only speak for myself (awaiting professional assessment, 29 F), but positive feedback is WAY BETTER than “constructive criticism”. I realise that sometimes that is necessary, but positive feedback makes me want to try harder and improve and I feel happy doing my sport, while negative (but well-meaning) feedback makes me feel sad and useless and like I’ll never be good at it and want to quit.