I've seen at the drycleaners a set of Urine destroyer and black light. With the lamp you have to go through the area to find the spots where they pee once you have located it use the liquid. Of course you can get it separately and choose the brands you like.
Here's a picture of the lamp
In any grocery store or pet store you can find the urine cleaner. Usually an enzymatic solution. If you are weary of chemicals you can also try a good ol fashion cleaning with a soft soapy solution and after that some good old vinegar at 50% and leave it there. Vinegar is REALLY efficient to fight odors.
Once that happened to my mom and we couldn't locate the spot so what I did is I just mopped the whole area normally and then with a clean mop with the vinegar solution. Your friend's place will smell like vinegar for a few hours but it will fade away and you'll end up with no smells at all. If it comes back repeat the operation a few times until completely eradicated.