Be sure the litter box is very clean, and lock the errant cat and her box and some nice bedding in a kennel at night. Probably need to keep her in her owner's bedroom and not down in the kitchen so she doesn't feel abandoned. Give her some extra attention also, because she probably is jealous. Kennels can be purchased in the US for about $50 for a nice sized one, don't know what that works out to in pounds. I have one that is collapsible and is handy for if I have to take them somewhere in the truck because it goes in the camper and is big enough for their box and a little leg stretching when needed.

SRS