One thing I say to fellow parents if they ask my opinion regarding ASD/developmental issues is “diagnoses don’t matter - but therapy matters”. IOW, it’s not so much if they fit the generalised label of ASD/ADHD/anxiety/OCD/whatever, but … what specific thing is the kid having problems with? Could a therapist help you with the particular problem you’re having, diagnosis or no diagnosis?
So even if these parents aren’t willing to go for a full-on assessment of their kid’s development, maybe they’d be open to seeing someone (doctor? speechy? OT?) about this specific issue, the lack of socialisation, which is clearly getting in the kid’s way.