Wait, he had to be their friend. Not their slave. He could still live at his own home and come by whenever he wanted. He didn't have to do whatever they wanted if he didnt want to.
The ONLY thing I could agree in the terms of their "friendship" is that he's not allowed to kill them until they die of old age. Maybe.
He could ask Mandy to do shit all the time if he wanted to, and if she refused he could have HER sent to Netherworld Court. Fuck, if he was clever he'd ask if he was her friend and if she said otherwise he'd just leave on the basis of her ending their friendship.
I think the only positive to Grim being their slave is that he's too stupid to realize he doesn't have to be.