Perhaps it's neither a urinary tract infection or blocked anal glands? JMTCW - we had a stray-ish cat that came into our house one time, he was a really friendly boy, and he settled in for a cuddle on my lap. When he jumped off, I noticed a horrible smell (I would also describe it as 'rank') and found out that he had a large burst abscess on his rear from a cat fight.

Obviously pus and urine are completely different, and look and smell different but we were always told at the vet clinic to consider all possibilities. Perhaps their cat has a burst abscess? Either way I hope that poor kitty gets sorted before it gets worse.