Re: Hello and help needed for father....
Welcome to the forum! So I gather that your Dad would mostly be pedaling when out on the flats, just using the motor to get there and maybe give himself a break from pedaling now and then? And no rel interest in going fast.
Since a China girl setup has the chain from the engine going round all the timethere is some noise and drag from that even with the engine off. A friction drive on the other hand can be completely disengaged from the driven wheel when the engine is off so there is no noise and no drag. So that's what I'd go for. I think dax has a setup like that. Someone else can steer you toward vendors. Electric is both expensive hand heavier with the batteries. Good luck and good thing you're doing for your dad.
Someday when I grow up I will probably lose interest in toys with wheels, but until then...