Can a non-autistic person watch an autistic person socializing, and, afterwards, give feedback, like point out the non-verbals he or she missed?

Physically speaking, of course they can. Anyone can observe, and then speak to the person they have observed - autistic people can do this too.Whether you should do this, though, is the question. If the autistic person has asked you to observe them and make comments to them afterwards, then it may be