Diagnosis in it self is not necessarily the end of driving, it would depend on the degree of impairment. The onus is on the individual to report such a diagnosis to the DVLA, and they can advice on supplementary testing (that said I’ve known people who despite clear recommendation from GP & Consultant that a person should not drive the DVLA have grant licence).
The Main problem is going to be the insurance company as they tend to look for ways have not paining out – they will need to be notified – but read the fine print first
Its (depending on circumstance/ situation) for the person not to drive alone – not at drive in busy times or in heavy traffic – its probably a good idea to have a companion as I’ve know people loose cars completely before now ( totally unable to remember where they had left it – found in the police compound six months later).
Has any medication been prescribed?
Who has made the diagnosis?
What support has been offered?
Feel free to contact me (email in profile) if you want more info advice