My fathers cat defecates outside the litter box?

My dad has a 3 yr old female cat the continually defecates outside the litterbox.the most common places are the rug in the living room and the one in front of the kitchen sink. she also does it in the spare bedroom.

On my last visit,i noticed she did have worms,so i did bring that to my fathers attention,i just dont know if its been taken care of,she is also not spayed and she lives with an older spayed female and a neutered male about her age.she obviously goes into heat quite a bit.

i am thinking of offering to take the cat to the vet for my father and have her checked out,get the worms cured and maybe get her spayed. she may be moving to my brothers home and he has 4 children so i dont want the kids to get sick because of her behavior.

i will say that i dont live with my father at all,so i have no control over the cat.so please no statements that I should take care of her better. my father took in the 2 female cats as a favor to my mother,and i dont want to see either of them head to a shelter(not a kill shelter anyway)

any advice on what could be causing her behavior other than the worms would be very helpful.