Some people do outgrow ADHD.

You can't diagnose your friend with it, the best you can do is convince her to go to a professional to get an assessment.

But ultimately it is her decision what she does. Until she is ready, it may be better to concentrate on other aspects of the friendship besides a diagnostic perspective.

And from what I have read, ADHD may be a risk factor for the development of Borderline Personality Disorder, in conjunction with traumatic childhood events/adversity.

Unfortunately a significant amount of people with mental disorders do not accept it or seek help. But if you press the issue too much you may do more harm than good.

Take care.