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Hi and welcome to JustAnswer. I'm Dr. Barbara and will be happy to be of help to you and your friend's cat, Kitty. Really, how nice of you to contact us on behalf of Kitty, and I'm so glad that she was reunited with her family.
It takes long for cats to defecate than it does to urinate, so when it is only defecation we zero in on litter aversion. In a two cat household there should ideally be 3 litter boxes in separate areas. This isn't feasible in some homes, so have as many as you can up to 3.
The litters should be in private but easily accessible places and should be kept clean as you've mentioned.
I'm assuming Kitty used the litter box well before, so your friend should use the same kind of litter and the same kind of litter box(es). Some cats are very finicky about the types. For example, if the light scent is new, then this may be Kitty's problem.
If and when your friend tries all of the above suggestions and Kitty is still using the bathtub, sink or wheelchair, then she may need to be confined to a small room with her litter and food until she is using the litter box regularly.
I hope these suggestions resolve Kitty's inappropriate defecation...they should. If you have any further questions or comments please let me know.