Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry for this concern about your friend's cat; I do have several thoughts as to what might be going on with him, though.
Typically, a stronger smelling urine odor, especially if the cat has been neutered, simply means that the patient is producing more concentrated urine...which isn't necessarily a bad thing, actually. I typically encourage owners of these cats to increase their water consumption since many cats aren't big water drinkers. This can be accomplished by either feeding canned food (since the first ingredient just happens to be water) or install a water fountain which many cats enjoy drinking out of better than a bowl.
As for the fur, the most common cause of thinning fur near the tail base is going to be:
1. Fleas although your friend may or may not see fleas since they can hop off and on a cat's body and they can be ingested from all the grooming. Cats can become allergic to the flea saliva such that they tend to overgroom or excessively lick this particular area of their body. So, I would want to rule this problem out just to be safe by use of a good topical flea product such as Advantage or Cheristin.
2. Inhalant allergies to such things as pollen, dust mites, mold, grasses, trees, etc. can cause cats to overgroom where the hair would appear thinner. These patients may or may not respond to antihistamines (see below) but they almost always respond to steroids which can help with the diagnosis.
Benadryl at a dose of 1/2 of a 25 mg tablet given twice daily or
Chlorpheniramine at a dose of 2-4 mg twice a day or
Claritin (Loratadine) at a dose of 2.5-5 mg/cat or
Zyrtec (cetirizine) 5 mg once or twice a day
It is important to ensure that the formulations used contain ONLY the antihistamine and are not combination products (e.g. Claritin-D contains pseudoephedrine, which could cause very significant adverse effects in a cat).
Sedation is a common side effect with these kinds of drugs.
3. If he's overweight and the hair/fur in this area was just more matted, then he may not be physically able to reach the area with his tongue to remove oils which are normally secreted by his body. If this might be the case, then I'd use a comb to remove the mats (if they aren't too bad) and wipe a warm washcloth (without soap) down his back at least daily to help remove some of those oils.
I hope this helps to provide possible explanations for your concerns about him. Deb