No, she shouldn't have grabbed him but given the situation I expect her emotions got the better of her.
I'm glad school are working with you. Keep talking to them because more needs to be done - if he's still distressed and lashing out its not being managed adequately.
Do they recognise this girl in particular is a trigger?
Ear defenders are good. Recognising that the girl and noise in general is a trigger and adapting environment/seating etc is good. SEN is good however the fact she only comes in when he's already distressed is not good enough.
He needs someone with him who recognises when it's starting to get too much and steps in, e.g. takes him to an agreed quiet area, BEFORE it gets too much.
Atm expecting him to cope and sending for SEN when needed isn't fair on anyone. Least of all your son. He's continually experiencing high levels of anxiety so he's like a balloon about to pop. They need to prevent the inflation not step in when he pops iykwim. 
Speak to the ed psychologist/ASD team/senco. Perhaps more can be done. New class? Increased 1 to 1? School with ASD unit attached?
I don't know. Just keep reaching out for help. I know it's hard. My friend is considering home schooling for her son who has ASD and struggling in mainstream.
I'm sure the right help for your son is out there it's just getting to it and knocking (and shouting at) the right door.
I'm sorry OP. It's tough but don't give up.