While I can’t blame you for being concerned, anonymous, it may be amusing to know that as malevolent as spiritomb reportedly is, it’s actually quite harmless thanks to the odd keystone that seals its spirits together. Putting it another way, imagine a sitcom with a cast consisting of 108 people who hate each other, all of whom are trapped in the same apartment for eternity, and that is essentially spiritomb. If anything, it’s more of a danger to itself than to its trainer. And even if it had managed to coordinate together for a single attack, a pokémon’s natural compulsion to work with a human being (or else attempt an escape), which is further reinforced by the poké ball itself, will keep spiritomb from attacking your relative.
Still, your relative should be aware of any poltergeist activity that may occur around her spiritomb. It’s extremely rare—and especially unlikely in this case, given the fact that your relative and her spiritomb seem friendly with one another—but it’s still a possibility.
In short, I doubt you’ll have anything to worry about, anonymous, especially if your relative already knows the risks of training a spiritomb.