Are you sure she has a *spelling* problem? If she can correct misspelled words on her own and she can spell long words, it seems she is capable of spelling.
ADHD certainly impairs output. My DS10 has ADHD and makes ridiculous spelling mistakes when he is composing in his head and writing by hand. Too many processes going on and spelling suffers. The more taxing the composition, the more grammar and spelling mistakes there will be. My son is very good at spelling and grammar when they are isolated. I am noticing that as he gets better at a type of composition, his grammar and spelling spontaneously improve. When he starts learning a new type of composition, his grammar and spelling appear to regress.
Perhaps reassurance that "I know you can spell because you can correct them" and "your brain has a lot going on when you write, I think it's great that you're putting so much effort into what you are writing." Encourage her to outline to relieve some of the demand on working memory. Encourage her to check and revise her work - you might need to be unconventional if she's resistant. My son despises the writing process he was taught in school, but he's a big fan of the cognitive-process writing process. He learned how to revise and became willing to revise after switching to a different writing process.
