Just opening the windows won't do it. The smell is following the urine, which has soaked into the flooring (yes, inside the pourous grout), the walls, etc. Use a product to deactivate the odors, as mentioned above. Another brand is NilOdor - something like that. Supposed to use bacteria to "eat" the substance causing the odor.
Nor will mere paint cover up the odor. Companies that rehab homes after fires, flood, etc., use special products to block the odor. I recommend KILZ, a fabulous primer, made to seal odors, stains, etc. It has killer fumes, though, so invest in a resperator mask. Either that or be very high after you finish working with it.
Yeah, I have known several people who like that "cave" dwelling. I could never understand why they'd have such heavy drapes, and have them pulled shut, during the middle of the day. They never turned on any lights, either - just the light coming from the T.V.... which was on 24/7.
I also know people who live in foul-smelling homes. Had a client just recently, the house REEKED of SICK dog. Plus last night's dinner leftovers stuck to the pots still sitting on the stove and sink. It's so queer to me, to see people dressed professionally, in suits and ties, going to respectable jobs, looking like everyone else, and then know they come home to a place like that. I think it must be underlying depression, that allows people to live like that.
Oh yes, KEEP their ENTIRE security deposit.