If you are not a professional detailer with odor removal experience, would suggest you either take the vehicle to one or recommend the clients to one.
Urine odor is very difficult to remove from many seats and especially leather, as the urine has penetrated through the stitching and has absorbed into the sponge backing of the seat cover and sponge seat. When urine is first excreted, it is on the acid side of the pH scale and very easily removed. Within hours of being exposed to oxygen it turns to a alkaline pH. When this happens it will increase in alkalinity to the point of becoming almost ammonia. Once it is dried urine contains salts that act as a desiccant. That is to say, it can attract and hold moisture better than most materials. If you pass a moisture meter over a urine infected area, even if it has been left to dry for months, you will find that the area where there is urine will have a much higher moisture content in relationship to the surrounding unaffected area.
This is the reason urine odor is stronger when the humidity in the air is elevated (rain outside/damp day) This ability to attract and retain high moisture in fabric creates ideal conditions for bacterial, viral and mold growths. Removing urine salts from contaminated affected area will eliminate the problem and restore the indoor air quality of the vehicle.
Since you cannot saturate the leather seat with products without damaging the leather or creating more of a problem.. You are going to have to remove the seat cover from the seat and treat the contaminated sponge lining of the seat cover (reverse-side) and the sponge for any odors..dry out and smell. Once you have solved the problem you can then re-install the covers. You also need to check the rear section of the seat cover and sponges and flooring as urine can leak back down into the back of seating and flooring/carpeting. Before you re-install the seat cover and once you have determined the odor has been removed ..you will need to steam the leather seat cover to open up the pores of the leather and apply an excellent deep penetrating conditioner to re-hydrate the area damaged by the urine or any harsh products you might have been using. In addition once you have taken the seat out of the car you also get to smell the interior away from the seat and this should tell you if the problem is only isolated to the seat.
Many will suggest the use of an ozone machine... I am afraid this is futile.. because you need to get to the source of the urine and REMOVE IT