This does not fall under reasonable accommodation guidelines, but rather reasonable modification guidelines. HUD makes it clear that the resident pays the cost of reasonable modifications. Because it's not for the resident but for their guest, it may be possible to deny it (talk to an attorney first), but if you treat it like a reasonable modification (which I would, were it me), then the resident will have to cover the expense of installation and removal. And if it allows your resident to enjoy their time more and doesn't cost you anything, then it seems like an easy decision.