This response is applicable regardless of the disability.
The primary question is one of safety. Will the person when qualified be able to operate unaided safely as a Solo Student / PPL in the system?
If the answer is no then the next question has to be - Is there any reasonable thing that can be done which when applied to the case would change the answer to the above question to Yes?
There are many pilots who the initial answer to the first question is no but when either they are required to use a modified aircraft / atrificial limb attachment or in other cases they are limited to flight in dual controlled aircraft with another qualified pilot the answer changes.
My initial thoughts regarding this and similar cases is that the person could have whatever aids they required to demonstrate the required standard. However, what ever aids were required to demonstrate the required standard would then be required at all stages and the licence would be endorsed so that those aids were always required to be used once the PPL had been obtained.
So being more specific, based on the question, this person will need some "aids" to demonstrate the required standards. Is it reasonable that the person could operate "solo" and as a PPL with these aids available and able to provide the required aid at all times?
Is not being able to do MDR / or use a Wizz Wheel a disability and should the CAA permit programmed calculators to be used to overcome such a disability?
Regards,
DFC