Edit: Opps, sorry forgot to include gregsully ! 😉
I think the problem is, is that the username is the one constant that can’t be changed - kind of the database key as it were. If your dad is really fed up with it, he could port it out to another network temporarily with a PAC code, and then port the number back into giffgaff under his own account, which would sort your problem, if slightly long-winded.
At the end of the day, giffgaff accounts and sims are free, so it is not really intended for an actual account to change hands, as there is no real reason for it to be needed (I do get that a sim didn’t arrive in time, which is why you did it!), although, in giffgaff’s defense, upon returning from the trip, your dad could then of got his own account / sim and number before he got used to your old one 😉