M.W.
I., it takes a few years of non-cat occupancy for the dander to be totallly removed from a house. It clings to the walls and floors. You are admirable for the lengths you are willing to go to for your son's future MIL's health, and your efforts will make things a lot better. My suggestion is to do what you intend, notify the allergic person what you are going to do, and ask if she thinks this will be enough or will her allergies still be a problem. If she is not sure, offer to have a nice dinner at a very good restaurant for her comfort and well being.
I have a lot of allergies, and I know that how many problems I have depends on how sensitive I am to the particular allergen. For some things, it's not a big deal, but I could never have dinner in a house where they keep a rabbit as a pet. So give the allergic individual a choice, then if she chooses to eat at your house and has problems, it has been her choice and there should be no hard feelings.