I think you should go to the hotel and change the name and the SPG no on checkin.,
the prepaid rate will probably already be out of your dads account though
so either get them to refund it at checkin and take from you cc or take the chance that they will still give you your points.
I find it is safer to get what you want done at point of checkin or checkout not over the phone where people get confused and I have seen bookings cancelled in Spain because of the smallest request.- and that was at an SPG hotel in Seville.

real problems re instating it!