If a car does not have a valid MOT certificate, it means the car is not roadworthy. You can argue that there's nothing wrong with the car but having an MOT is a legal requirement and there's a clause in every car insurance policy which states that your car must be roadworthy to be insured. So technically speaking, your friend was driving an uninsured car as the insurance company could invalidate her insurance because her MOT had expired. She could be left paying for the damages herself and also be prosecuted for driving without mot and driving without insurance.
A while back, insurers did not check whether your MOT was up to date but nowadays they tend to check everything before processing your claim and paying out. If luck is on your friend's side, then she could get away with her insurer not finding out about the MOT thing.
