I know that I've entered this thread late, but your relation's insurance will be voided as soon as her insurers find out that she hasn't declared her health problems. Most insurers have the following, or similar, clause in their Terms of Business:
"Disclosure of Information
It is your responsibility to provide complete and accurate information to us when you take out your insurance policy, throughout the life of your policy, and when you renew your insurance. It is important to ensure that all statements you make on proposal forms, claim forms and other documents, are full and accurate. If a form is completed on your behalf, you should check that the answers shown to any questions are true and accurate before signing the document.
You are reminded that it is an offence under the Road Traffic Act to make any false statements or withhold any relevant information to obtain an Insurance Certificate."
I would strongly advise you to ensure that her insurers are informed immediately, and if they are subsequently willing to cover her, OK. Otherwise get her off the road as soon as possible!