So I’m gathering - this woman asked one of your relatives to take her some drugs, your relative did so, you were annoyed at this and went and shouted at/threatened the woman who originally wanted the drugs.
The police really don’t want to be dealing with these silly squabbles if I’m honest - it’s all he said, she said and rather petty.
As your relative was supplying the drugs by giving them to this woman then it that is an offence, and I don’t know how it’s been dealt with but hopefully, it’s taught your relative a lesson. If she’s autistic then the police would be aware and that can all be taken into account at the station, by the CPS, or in court if it goes there.
It is your relative who has to learn society standards, what happens when you don’t meet them. She also has to learn that not everyone is a genuine friend. Threatening everyone you think takes advantage, and after the events anyway, is not helping your relative learn how to consider these situations in advance - perhaps you could put your energy into that.
It’s done now with the police - if you keep it up and piss them off really then they won’t be as generous next time.