'Do you really think a DC with practically no friends who spent virtually all her leisure time alone would not have some people questioning nowdays whether she had SN - rather than the fact that it is just part of her personality.'
Some people may, nobody relevant though - if she worked fine with others in school and her parents knew she played alone through choice, who would be raising it as an issue?
Some people seem to think (I'm not saying you do, I wouldn't know) that getting a diagnosis of an SN is easy, it's really really not... Nobody gets a diagnosis just because they're not sociable, nobody gets one for being a bit odd.
My DS was 7 when he was first assessed, we came out having being told that yep they strongly believed that AS was a possibility and that they'd see him again in 6 months and they assessed him again then, and 6 months later, then by a different team 6 months later, then back to the original doctor, then a clinical psychologist and so on until he was finally diagnosed at 13 - and all that time he had SALT and OT involvement.
None of them, or me, or his school had any doubt after that initial referral that he would be diagnosed with something and the older he got the more obvious it became that he wasn't a typical child, the problem was that he also didn't quite have a typical presentation for AS, so they don't diagnose if there's any doubt.
All the time he was undiagnosed, his school were receiving less funding for him than they would have been if he was diagnosed, he didn't qualify for any autistic only provision and he didn't qualify for anything outside of school either.
On leaving primary school, he'd never been on a school trip that involved staying overnight because both me and the school agreed that he wouldn't cope, he'd had no friends for years and was so unhappy about it that he'd stated several times that he wanted to kill himself, he had a reading age of 2 years above his actual age, but a spelling age of 5.5, he couldn't write well enough to read it nor tie his shoelaces or fasten most buttons. On top of that he has a speech disorder, which affects how sounds are made, making him hard to understand.
He was not a bit odd, or different, he had fairly major problems and very obvious ones and yet he had a diagnosis or 'label' of absolutely nothing - because he didn't quite fit the diagnostic criteria.
I've done a few years of voluntary work and I've worked in schools, he is not a one off, I've met many others in a similar position, it often takes years with higher functioning children like my DS to get a diagnosis.
So no, I don't think anybody with any authority would have seriously questioned whether your friend had an SN just because she prefers to be alone.