Imagine Autism/ADHD as a spectrum, for a very simplistic example imagine a ruler running from whatever is the opposite of Autism (perhaps not autistic for this purpose) to profoundly autistic, or for more nuance a globe shape encompassing spectrums of Autism, ADHD, introversion and whatever other personal characteristics.
The place on the ruler (or the parts of the globe) where someone tips over from "not autistic" into "autistic" are an entirely man made construct.
So, for example, it could be easy to get a diagnosis of autism for someone with echolaic speech, lines up toys, still in pads as an older child but even experts could disagree about whether someone should have a diagnosis if they are in a grey area.
Personally I think that autism/ adhd are not one thing but many things. I imagine this like someone could have a presentation of a "sore leg" but this could be caused by tens if not hundreds of different things.
I think presentations of autism could be caused by thousands of genetic variances, environmental factors/pollutants, social environmental factors, trauma etc.
Viewed from this perspective it is quite subjective whether someone in the grey area is "autistic" or "not autistic" and whichever side they fall on those people are just variations of "normal".
Personally I don't particularly believe in neurodivergent/ neurotypical - I think we are all complex mixes and we would be better off thinking about people who cope well with societal expectations and people who find it more difficult to manage societal expectations.
Questions to consider for your mum are:
Would a diagnosis improve her life?
Would medication (with or without a diagnosis) improve her life?
Would any other intervention such as therapy be welcome or helpful?
Can your mum's need for alone time/certain rituals be accommodated to make your mum more comfortable?
Is she happy as she is or does she want to change?
** If too long didn't read: if you mum is happy let her get on with it. If your mum thinks a diagnosis could improve her life ask her to see her GP about this.