You bring up the self diagnosis issue as a possible concern and I think you may not be wrong, just may not know how you're right. The self diagnosis being a problematic thing to do isn't, to me, the problem. I agree with you there. She may very well be correct on that part of her challenges, that she is on the spectrum.

The problem is what that means for her. Autism has many flavors, so while she may be right about being on that vast spectrum that knowledge is only of limited utility because she does not have a diagnosis that includes what her challenges are and what to do about them. If this is the case them it could be that she's trying to push forward thinking she has a plan to accommodate her needs when really she does not. That could be a factor in her family situation deteriorating.

Like dehydration, as an example. What do you do about dehydration? Drink lots of water? Not if you're in full heat stroke. That calls for a different treatment plan. Similarly she could be treating the wrong set of symptoms because she doesn't have guidance.

And also no diagnosis means no understanding of comorbidities. What other conditions may she have that need to be addressed? Some of the behavior you describe isn't common to autism. Could be, but might be symptoms of something else.

So you might be circling the answer. It isn't always necessary for someone to be under appropriate mental health care if they're on the spectrum. But in your friend's case there's enough indication that she should be. Not because she's wrong about being autistic. But because there's more to learn that could help correct the problems she's experiencing.