This.
And better still, apply to be her appointee. This means that for all assessments, you can speak for your dd and she does not have to say a word.
The PIP assessors will try and catch her out and draw unfair conclusions from the things she can do. So you have to be very careful what you say about dd as they can twist your words. For example, if she is in university, the assessor says she must be able to manage her finances when in actual fact, her finances may be a mess because she is distressed and unable to focus. Also, if she can drive, according to the DWP, she can do anything. The assessors can blatantly ignore your evidence that she cannot manage.
Hence you have to go for mandatory consideration and then appeal. It is only on appeal that your dd will get a fair hearing. But she must come within the strict PIP criteria and your application must specifically address the criteria.
My dd has autism and she has PIP. I was her appointee and absorbed all the stress of the application for her. I read up loads and consulted an independent PIP adviser/charity (do not ever get advice from the DWP). My dd finally got PIP (enhanced award) when the DWP settled just ahead of the tribunal hearing. But by then, I have already got my evidence ready to go.
This is a comprehensive guide to filling in the PIP form, gathering medical evidence and challenging PIP decisions, including for autism:
https://www.benefitsandwork.co.uk/personal-independence-payment-pip/pip-claims
It costs 19.95 per year but has detailed sample language throughout, including how to ask your GP for a letter. I read a lot of online guides, including CAB's. This one really worked for dd because it literally tells you how to write it.
It was also recommended on mn.