If a person drives with an untreated medical condition that creates a danger for others then that person would be responsible for a collision caused by that condition. For example, if someone faints on a regular basis, and then gets behind a wheel of a car and faints, whoever they hit would be able to recover against them.
A valid CDL requires a current medical card to prevent just this sort of a collision. It sounds like the doctors wont give a clean bill of health for the card even if the state license has yet to expire. The regulations on driver medical requirements can be found in the FMCSR's governing CDL's and are easy to find on the web. In short if the Doctor's are competent, everytime your family member gets behind the wheel they are endangering themselves and everyone else on the roadway. The loss of mobility may be painful, but a loss of life will be more so.