I don't have any solution other then what you are already doing. I think it was incredibly rude of her to say to you what she did and I am not particulary a rattie fan. But is it possible she just is trying to find a way to NOT come to your house?
In my house the guy I bought it from - his daughter lived here and was a drug addict (she has since cleaned herself up) and had a cat that used the floor as a litter box. When he decided to sell the house - he ripped up the carpet and painted KILZ all over the floor http://www.kilz.com/ and there is no smell. Course this is a bit drastic.
When I was potty training Keegan she kept going to a spot in the floor where a floor furnace had been. We ripped it out and my dad put plywood there.
I kept a rug over it and she just kept peeing on that spot. Finally I asked my parents when they came over to paint Kilz on it and they painted like 2-3 coats. Then we put some carpet over it. She never peed there after that!
My mom has a super sensitive nose and usually when she comes to my house she doesn't smell anything. The last time she was here she said she smelled cat pee. I of course didn't smell anything. I changed the litter in the box but I didn't smell it. One of my friends when she feeds my cats she always asks me how come my house doesn't smell like cat pee - even in the closet where the box is. I said I dunno. It isn't pee that I smell usually its the poo.