Depending on your location and how much support her type of Autism requires, it may or may not be possible and/or worthwhile. For reference, I was in my early 40s when I started to consider I had it (it has been known to be in the family) and I live in a major city, though not one of the largest ones in the US.

First, there are few providers who are willing and able to provide a diagnosis for an adult. The methods for doing it is not well known or tested. It was thought in the past that the type of autism that used to be called "high functioning" or now are classified as low support need, actually went away with age. Turns out the people just masked and developed coping mechanisms. And the older a person is, the more difficult it is to diagnose due to the coping mechanisms, masking, and the symptoms having other possible and more statistically likely causes.

Second, there's no standard treatment available for adults with this type of autism. Arguably, the treatments for children aren't effective either considering they basically just teach the few kids who are diagnosed and treated how to mask.

Finally, the testing for it is extensive and costly. And since there are few providers and it's so difficult, many of those providers are booked solid and have no need to accept low paying/high cost to bill insurance. So finding a provider that is both locally available and in your insurance network is generally extremely difficult and even if you find one, likely to have very long wait lists and probably still require significant amounts of money for tests that insurance doesn't cover as well as coinsurance and deductibles.

So, I've personally given up on a formal diagnosis. There is a university-associated office that has a couple of providers who do it in addition to treating children and are in my network, but the wait-list is about 5 years to get an initial consultation. But my therapist and primary care provider treats me as if I do have it even with the self-diagnosis as they know how difficult it is to get a formal one. Also, my friends who have Autism always thought I did and thought I knew I did, so that is what stared me thinking about the possibility. And it turns out neurodivergent people tend to find each-other as they find it easier to make friends with other neurodivergent people, so it's a good indicator that if a bunch of your closer friends are, then you might be too. 🙂 And friend support and tolerance of your differences is more important than anything, IMHO.

And I've also always worried that a formal diagnosis may or exclude me from some things in my future like employment or immigration or other things that behaviorally disabled people can be excluded from legally, so for me, it's never been worthwhile. But if you're trying to get her some kind of disability benefits, that may not be a concern for her case.

Not trying to disuade you, just want to give the things to consider while mentioning why I chose not to. I did get a formal ADHD diagnosis because it is treatable with medication, but since autism isn't, at least not yet, it just never was worth the time, effort, and cost for me.