If involved in an accident, the insurance couldn't wriggle out of the third party coverage, so he would be legally insured. However they might argue over repairs to his car.
The cops could catch him quite quickly though, as ANPR (number plate cameras) will flag up the lack of MOT.
It is possible he is still covered by the previous year's MOT, if it hasn't expired. This depends though on why it failed and if it makes it unroadworthy.
thanks Hoppy for that detailed answer - it is quite an old car and of course (I don't know him too well) but instead of making arrangements in getting the repairs done he is still driving.
I did know that about the cameras and it would be awful but perhaps he needs to learn the hard way. thanks again