so there is a separate relay on the circuit that could be weak??
There is a starter relay, I will go down to the shop and check my manual for the location on my 94FLSTC. I think it is under the seat. Is the problem you described with your dads battery the same you had with your old starter?
the relay could definitely be the problem from what I see written in the manual. Is there a quick way to access the relay without removing the oil bag or pulling off the back wheel to remove the plastic fender extension that the relay is pop rivetted to?