There can be types of dizziness that are ear related that are not positional vertigo. Has she had a VNG (video nystagmography)? This is an in-depth balance test that checks for other possibilities. (Neurologist may have done this.) If they want to further investigate any ear-related possibilities, they could see a neurotologist. These are further specialized ENTs who deal only with ear related issues. Basically, combo of neurologist and otologist.
I have worked in ENT practices for 12 years and never had a patient who was so severely dizzy that they needed a wheelchair for an extended period of time, so this is obviously something pretty complex and maybe something that her ENT has never seen before. I would see as many different doctors as possible to maybe find someone who has experience with this type of case. It is impossible for even a very good dr to have seen all rare and complex diseases/disorders.
Of course, this is obviously all dependent on what insurance will approve/cover.