I would classify this as more of a health issue than a safety issue.
My first move would be to contact me client and inform them of the situation.
So long as they don't sleep on your face and smother you to death like Huple's cat did to Hungry Joe in Catch-22meant to say health in my post but after researching it cat odors do not pose a health issue as far as I could determine.
Rip the carpets out, soak the subfloors with a massive amount of bleach and, after the subfloors dry, coat the subfloors with oil-based kilz primer and the cat smell will be gone forever (I know as a I rehabbed a cat **** house that I bought out of a bankruptcy in 2007 or 2008 and own as a rental to this day) However, if it has gotten to the point where the subfloor and/or floor joists have rotted, then removal and replacement of the rotted materials is necessaryOnce the urine has soaked into wood floor, or subfloor, you have to tear it out to get rid of the odor.
Back in the 90's, a friend bought a REO owned by cat lovers, she had to pull carpet & subfloor t/o the 1st floor,
and, worried that she should have replaced some joist. Nasty stuff, some of the plywood was disintegrating.
I wouldn't ignore the situation, you might want to ask for a specialist -of some sort- look at the property for damage.
I'm curious; What was under those carpets? plywood, flakeboard, hardwood finish floor, concrete ?