Is there a way to have a person stay dominated by the PC forever?

Spell cast ba anyone, same as Dominate with parmament duration?

what exactly are you trying to do? are you building or playing?

If you're building a mod, just make the npc a henchman.

If you're playing a mod where you want some dominated npc to stay in the party, I think you're out of luck unless you want to go into the mod and mess around with it.