I think, whatever your father says the motorcyclist was doing, they're going to take a dim view of a driver hitting one. Driving *with* due care and attention means being aware of what other road users are doing and avoiding them. After all, people can't defend hitting a pedestrian if the latter steps onto a pelican crossing on a red light. Quite what the court will say though, I really couldn't say. If there is any doubt about his fitness to drive, he'll probably be asked to undergo a medical and/or a driving test.
However, if your father ends up with points on his licence, he will be legally obliged to tell his insurance company, and they may well decide, given his age and this case, to either refuse to insure him or to raise his premium to a ridiculous level. Perhaps that will stop him.
I take it you have pointed out all the benefits of not driving. The expense, the worry of being 'responsible' and the sheer headache of driving in traffic will all be gone. As for your mum, many buses now are capable of taking wheelchairs, and what your dad now spends on running a car can be used instead for taxis.