The GMC guidelines (reproduced with permission) state:
The driver is legally responsible for informing the DVLA about such a condition or treatment. However, if a patient has such a condition, you should explain to the patient:
That the condition may affect their ability to drive (if the patient is incapable of understanding this advice, for example because of dementia, you should inform the DVLA immediately) and,
That they have a legal duty to inform the DVLA about the condition.
If a patient refuses to accept the diagnosis, or the effect of the condition on their ability to drive, you can suggest that they seek a second opinion, and help arrange for them to do so. You should advise the patient not to drive in the meantime.
If a patient continues to drive when they may not be fit to do so, you should make every reasonable effort to persuade them to stop. As long as the patient agrees, you may discuss your concerns with their relatives, friends or carers.
If you do not manage to persuade the patient to stop driving, or you discover that they are continuing to drive against your advice, you should contact the DVLA immediately and disclose any relevant medical information, in confidence, to the medical adviser.
Before contacting the DVLA, you should try to inform the patient of your decision to disclose personal information. You should also inform the patient in writing once you have done so.