I suppose that I am a bit biased as my brother and his wife are both vets and tell some pretty wild stories about dealing with pet owners who shouldn't be. Of course there are always three sides to every story as they say -- his, hers, and the truth. And when it gets to anything dealing with sexuality -- well people can get a little wierd (there was some discussion on here of this recently regarding depriving animals of a satisfying sex life.)

I suspect this woman misunderstood. Perhaps her vet told her the cat should not be spayed WHILE in heat. What I might do is get some articles about spaying and give them to her saying..."After we talked about you cat the other day I realized there's alot about spaying that I don't know so I did some research. What I found is really interesting and I thought you might like to read it too." (I realize you DO know about spaying but, as she seems defensive this may make her more accepting of the information.)

Good Luck!