Memory problems in most areas but can drive for short journeys - are these incompatible in PIP

MY friend has multiple conditions with significant bowel, incontinence, one-side weakness and Crohns disease, severe anxiety and some physical disabilities. Over the past 2 years  he has experienced severe problems with short term memory - to the point that dementia is being investigated, and he needs prompting, reminding to do things, has to make lists for everything,  and has increasing problems with dealing with longer texts, any bank statements, bills, etc., becoming overloaded mentally and getting distressed regularly about simple things. 

Despite this he is able to drive familiar short journeys independently, in an automatic car and with satnav, which I know is not an accepted aid. He can do this safely and reliably and needs no help to do it, perhaps because it's a longstanding skill and not dependent on short term memory. He can't make longer journeys without support, prompted directions or several practice runs.

He has a lot of very genuine daily living difficulties, needs help and assistance a lot, and especially more and more with prompting and reminding and needing to have simple cognitive tasks done for him or with him, because of memory or confusion issues, in addition to extensive digestive and continence issues which keep his housebound a lot of the time. 

  He's not expecting to receive any mobility component because of this, though he has other physical impairment, but does the  fact that he can do these simple drives to familiar places undermine his  claim to to the daily living component? Is an assessor likely to say, well you can drive yourself, so you can't have any cogntive impairments, or real memory problems. Is his driving ability likely to endanger his claim for the daily living component of PIP?