Wow, that's a tall order 
Look up the PIP advice pages on the CAB website, that shows you the questionnaire the parent will be completing for them.
www.citizensadvice.org.uk/benefits/sick-or-disabled-people-and-carers/pip/
As you'll see from that, PIP isn't awarded for any particular condition, it's awarded for areas the claimant needs a lot of help and support with.
www.citizensadvice.org.uk/benefits/sick-or-disabled-people-and-carers/pip/help-with-your-claim/your-supporting-evidence/
As you've said student, I'm guessing it's mainstream school?
I've used 'her' throughout my reply as I'm thinking of educational 'stuff' that pertained to my daughter or was enquired about at various assessments. Your student's needs may be very very different, every autistic individual is different in terms of strengths and deficits, but this may give some idea.
Clearly outline all of the differences between the claimant and her NT peers in the school environment.
What can she NOT do that they just get on with without a second thought? Things like
Find the classroom on her own
Have the right books and equipment every time
Arrive on time and not be flustered
Sit still and listen
Speak confidently and ask pertinent questions
How does she process information, does she need specially adapted instructions from you one at a time to do a set task (whereas her peers would only need a single sentence and 'intuitively' know what to do)
Does she need to be close to the board to keep focused, is she easily distracted, does she interpret things literally thereby missing the point.
Is she very easily overwhelmed by the amount of information she's given to process, or by what she feels are expectations of her?
Given a block of text can she pick out the relevant bits by herself to expand on or does it sail over her head and she focuses on the details but completely misses the bigger picture?
Does her anxiety level affect her ability to communicate or work effectively?
Can she do a task one day and be absolutely stumped by the same thing another day?
Does she need noise cancelling headphones in order to concentrate or any other aids. Can she keep up with writing speed for taking notes or does she need a laptop or printed sheet?
Does she need a 'buddy' to get to lessons, how is she at break, does she find socialisation difficult (some autistic girls are very sociable, look up the term masking)
Is she emotionally immature compared to her peers? Neurodiverse kids are often emotionally two thirds of their chronological age.
Anything that pertains to your student's differences in the classroom and wider school environment will be helpful.
I know this sounds horrible and goes against all our instincts and manners, but please don't say things like 'she's a lovely popular girl who just needs a little bit of help.'
Do not minimise any of her struggles, do make sure you write the truth but make sure the extent of those struggles can be understood by the DWP assessor and if necessary (and it often is!) a Tribunal Panel of Judge, Medical Expert and Disability Expert.