Granny I'm glad they recognised how unwell your dd is, I'm surprised they didn't arrange admission to a peads ward today tbh, did they check her obs? 76% wfh is so low bless her, don't hesitate to call 999 if she deteriorates.
Little I suspected autism with my dd pre ED, she had significant sensory processing difficulties from a young age, she had friends but always on her terms, had frequent melt downs, very young for her age etc.
Frustratingly she was assessed around the same time she developed the ED by deaf Camhs (she had hearing loss when she was younger) and they decided all her issues were due to the hearing issues.
It's difficult if they have only developed traits when they are in the grips of the ED as anorexia can present very similarly (obsessive, rituals, black and white thinking etc) I'd be reluctant to accept a diagnosis until they're at least weight restored and eating okay.
I'm not sure it changes the treatment much anyway, I think you can tweak certain things so there's a sense they have more choice (they can pick between two snacks of equal calories for example) and there might be some allowances for sensory issues with food/textures etc.
I played to the autisms strengths of dd needing routine and predictability, she loves a routine and rule so I used that when implementing FBT.