Blueberry always sits and sleeps on one of our chairs too, like its his chair. So I know how you feel, I would hate it if my parents decide he wouldn't be allowed on there anymore.
Sorry, but I don't really have any advice for you. Try to explain to your husband that Tasha will not understand if you just suddenly tell her that she can't do something that she's been doing for so long. She will just get really confused.
I hope you find a way to solve the problem soon.
Good luck!