Hi, I have some experience with this problem with my own autistic daughter. She had severely matted hair at one stage, and I managed to get it all out myself. It's a long job, can take hours, so lots of patience needed, and breaking it up over days.
So, I used a spray on leave-in conditioner. Spray generously, just concentrating on one small section at a time. With a very large solid matt, just work on the bottom couple of inches. Let the conditioner sit for about ten minutes.
Tangle Teezers & combs are crap for this, I use a Denman classic style brush. Then holding the hair, and the tip of the hairbrush (holding it vertically), I would start with the first centimetre of hair. I hold the rest of the hair to stop my daughter from feeling the pulling.
I literally took it centimetre by centimetre from the bottom up. I've never let her hair get that bad again. She thrashes about a lot in her sleep and it can become matted overnight, so if her hair didn't get brushed for a week, that's when the severe matts came (please no judgement, if you have never experienced an autistic child with long hair who refuses to let you brush their hair).
Anyway, as she's gotten older, she now understands that hair brushing is now non-negotiable, same as teeth brushing (we had issues with refusal of that when younger too but thankfully overcame that and her teeth are all healthy).
It's a shame I don't live close to you - otherwise I'd come help. Honestly, I've never had to cut any hair off and I've always managed to get every mat out. It just takes a lot of time.
If I've not been clear with the method, feel free to DM me.