Our society's expectations are built on a person's ability to be able take their keys, start their motor vehicle and drive to wherever they please at any time. This is an incredible freedom and one that we seldom really appreciate until something like this happens. One can no longer travel on the spur of the moment and we have to bring along a companion to look after us. I suspect that one's sense of self worth suffers when this happens.
The question of liability for the chaperone is a good one. I once managed two impaired charges out of one vehicle when I investigated the driver for driving and the supervisor for being in care and control of a motor vehicle while impaired by alcohol. Both were convicted. Clearly, the criminal law holds the supervisor to account and will punish those who fail in the job.
If you are going to supervise this driver it would be wise to visit an Autoplan Agent and review the liability coverage for the vehicle being used. In the event of a collision monetary damages are looked after by the vehicle insurance, ultimately settling on the vehicle owner via the driver. However, if the driver and owner do not have sufficient coverage and assets, it is possible that you as the supervisor could be subject to legal action to pay for losses suffered by others involved.
Finally, do you have sufficient knowledge and skill to supervise another? I often ask the question "Have you ever met anyone who would admit to being less than a better than average driver?" An honest evaluation of your own capabilities might be in order before you volunteer because this driver is depending on you to help them drive safely and properly.