I'm auhd and so is my daughter,
What do you say when she brings up this girl?
You need to not engage about this girl any longer. Your dd is obsessed at this point isn't she, ( not meant in a nasty way, from.my own experience asd people to tend to get obsessed with people or their perceived actions )
You need to just disengage.
My dd is very similar but in the sense of being picked on. It's normally not. It's tit for tat kid stuff...... but, if I give her too much attention about anything school wise, she runs with it. She will come out talking about whichever child has upset her, for days, she looks for slights ect
Just disengage and be very straight, I'm not talking about this with you anymore. It's been over a year now and nothing has changed.
This girl doesn't live in your house, isn't part of your family, why is she taking up so much headspace from you and your dd?
She's obsessed over her this long because essentially you have let her. Both of you need to pack it in and just stop talking about her. Distract. Change subject ect