Pete, the best advice so far is from your customer. Forget all the other nonsense, put the carpet on the floor and rinse and rinse and rinse. It is the only way you will get the pee out and any odour along with it.
I understand it can be a bleeder but if it was going to bleed it would have already done so.
Solvents will do next to nothing in this instance. And dry compounds arent worth the time and or effort as they simply wont correct your problem to a 100% "clean".
If needs be, put it on a slope and let the hose run over it. For hours if needs be, similar to a nebulus (spelling) type of clean.
Best, Dave
P.S. Ken or anyone, why would you even entertain to vac a rug that has urine in it. It would then take you longer to clean and sanitize the vac than cleaning the textile. Pee stains should never have a vac put over them. Of course this is just my opinion.