Is she really, really sure that resignation is the best option for her? From a tactical point of view, she might be better off not resigning, and leaving the ball in their court.
Unfortunately, the obligation on employers is quite low. They are obliged to provide any 'reasonable accommodation' required to accommodate her disability. The Courts will consider the size of the employer in deciding what is reasonable, or not.
She should probably involve her trade union, or the [broken link removed].