What can a student in England do, if her Disability Office refuses PDF versions of textbooks?
I have an alternative take on the circumstances.

From Wikipedia, a Print disability refers to an impediment that prevents the reading of printed material. Presumably, the PDFs the disability office has are intended for students that need to use some other method, for example: 1) a text-to-speech program, which can't operate on the printed books, or 2) a digital display for a student with a disability that prevents them from turning pages.

It sounds like your sister has trouble carrying physically demanding loads, like a heavy textbook, but that doesn't mean she can't read a textbook that is sitting in front of her on a table. If this is correct, indeed your sister may not meet the definition of "print disability" that the DO is using.

Although the publisher sent your sister to the DO to obtain PDFs, they probably want no part in assessing whether the accommodations are necessary, they are punting it to the DO. The DO, on the other hand, might have a specific licensing agreement with the publisher that only allows them to provide digital copies if certain criteria are met under their definition of "print disability." It is possible that difficulty with the weight of a textbook doesn't fall under their terms.

I would suggest meeting again with the DO before escalating this to other offices, explain the nature of the disability, and see what accommodations can be made. Personally, when I was an undergraduate student carrying around heavy textbooks just wasn't a good idea for anybody. They were rarely actually needed in a course, they were used at home. It might be that the DO would rather provide a physical textbook copy in courses where a physical copy is needed in-class on a course-by-course basis, rather than providing PDFs (which they may not have a legal right to distribute).

Alternatively, as others have suggested, your sister could obtain digital copies on her own if possible.

(just to add: I think it's unfortunate, to say the least, that the DO didn't directly suggest other accommodations. You have a right to be frustrated by that, but I would try to be polite in the short term and try to assume that the DO head is simply busy rather than negligent. DO's could very easily be understaffed. Maybe they truly are just a jerk, but if they aren't and you assume they are, it will make the situation more difficult for everyone. Assume the best, try to be proactive, try to be assertive without being demanding, etc.)