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Hello, Welcome to Just Answer and thanks for your question. She can contact any psychiatrist or her local mental health clinic for testing to make sure she still has ADHD and/or ODD. Many adults still do continue to have problems all their lives.
Unless she is a danger to herself or others and her rights have been assigned to a guardian, she is presumed to be able to make her own decisions. The whole family can try to persuade her (some call it an "intervention") but unless you are willing to start legal proceedings, there is no other way to force her to get help if she doesn't want to.