Your Dads room size can make a big difference. If he is going to be using it in a small room, the 8" sub may be more than enough. IMO, a big sub can be sometimes overpowering in a small room, especially for music. Likewise, If he is going to be using it in a large room, it may not be enough for him, he might want to move up to a 12" sub. If they will let you return the smaller sub after a few weeks if you don't like it, then I say try it out and see how he likes it. If it's not enough, then you can upgrade.
Jack