Awful. Being autistic she may well have meant it at face value without it being an insult, and not understood what she did wrong as she just stated a fact. Then being dragged out while she had an autistic meltdown :(
Even if she did mean it as an insult and wasn't autistic, there'd be still no need to drag her out and arrest her. She's a kid. Educate her, sure, but there was no need to escalate it.
If this girl is so far gone she can't stop herself from accidently being homophobic, why were the parents letting her out to drink on the street at midnight?