If you know where the assessment is taking place it might be worth ringing them to talk about the process involved. I looked into this 3 years ago. The nearest assessment centre to us was 35 miles away and is a private centre that is licenced to do reports for DVLA. They also do assessments for anyone who wants this for themselves to help them return to driving, after a long illness for example. They were really helpful when I discussed it with them on the phone.
At the time MIL wanted to carry on driving against her family's advice and she thought she could have an assessment to get a few tips about getting her driving skills back up to speed. The problems MIL was experiencing were with judgment and processing of information, as well as visual processing following cateract surgery. That and not being able to drive more than a mile from her house because she would get lost. For various reasons she decided not to pay for an assessment. Her GPs were not very helpful, just advising her of the risks to others, blah, blah, which she just ignored.
Your friend is fortunate in having a consultant give an opinion because this helps DVLA to take action. If a specialist has notified DVLA I would say that the time for safe driving is probably past for your friend. If she wants to go through the assessment it may help her to accept this, because it will be 'official'. The alternative to being assessed would be to voluntarily surrender her licence to DVLA.
As I recall, the assessment centre said that it isn't always a pass or fail situation; sometimes they can make recommendations that will allow DVLA to issue a temporary licence with conditions on it, so the driver has more stringent monitoring than most drivers do.