Get one of the pet stain and odor products and spritz down the tub whenever you walk by. The enzymes should help to fight back the smell, at least some.
Otherwise, I do think there's any health hazard from just the smell, though the ammonia from the pee might trigger allergic reactions.
Thanks for the vote. I can tell you are not wanting to help the guy you are mad at, but if your boyfriend won't have the talk with the renter, you have two choices. One is to sit and stew and be furious at both the renter and the boyfriend, which in the end makes you feel impotent and at their mercy (sitting around with a big mad on doesn't do any good, your stomach lining takes it and your boyfriend probably hears it as nagging). The other option is to take action. You could get a big, black contractor's trash sack, put it over the litter box, tip the litter box over into it, seal it off, and put it into the trash. Then spray down the box with Nature's Miracle or even Windex, and for the sake of the cats, put fresh litter in the box. After that, tell your boyfriend that if the roommate ever lets that happen again, you are not emptying it again, you are leaving. Or simply tell the boyfriend that he is such an indifferent wuss that you are leaving anyway. Maybe he likes the roommate more than you think and hasn't been willing to jeopardize the friendship. He needs to know the price he'll pay if he isn't willing to have the talk with the roommate.
If your boyfriend he owns the house, is there a clause in his agreement with his renter that the renter will abide by rules appropriate to keeping up the value of the house and all? A huge bin of used cat litter is nasty and not appropriate to have in a house.
If the renter refuses to take care of the problem, he should look for a new renter, or if he is disinterested in talking with him about the issue he should just clean it himself.