I gave him the link to this thread, I am sure he will find it helpful. This is how I replied... (I probably should have included this info earlier, but it is reassuring to see that others have similar feelings on the topic):
My best advice: Remove the food, and the other cats' litter boxes and keep the other cats out of the basement. Cats ideally only eat twice a day, morning and evening, so not leaving the food out won't hurt anything. When supper time comes, there are two options.
a) if your sister wants to invest the time in it, she can sit downstairs with a small plate of food (usually a strong smelling canned food is good for this) and try to bribe the cat to come to her for the food. It can take a long time - and the cat will probably be able to out-stubborn her if she's not very patient.
b) if it's more of a priority to get the cat out of the basement sooner rather than later, she can put the strong-smelling canned food into the haveahart trap and catch the cat that way.
Once the cat is caught, she can put the litter boxes and food back downstairs for the other cats, and her best bet would be to isolate the new cat in a room of its own. Continue feeding twice a day, and ideally sit in the room with the cat while it eats. The cat will come to associate her with good things, like food, which will make bonding easier.
Then it is best to introduce the new cat to the other cats slowly. She can start by taking a hand towel and rubbing it on the new cat, then rubbing it on the other cats so they get used to the new cat's scent. Another trick is to let them interact by playing with a toy under the door.
Once they get used to each others scents she can begin allowing supervised excursions from the "safe room" for about 10 minutes at a time, a few times a day, and gradually increasing the amount of time they are together, as well as the frequency of interactions.
Here's a website that explains introductions in more detail:
at this point, my friend replied: "She's already tried the Havahart trap - it got set off but the cat wasn't inside, so I'm sure it got spooked and won't go anywhere near it again."
you'd be surprised, but cats can be trapped multiple times, even though you'd think they would avoid the trap like the plague... and the trap is definitely the quick way.
If I were her, I would remove the other cats, all but one litter box, and the cat food first thing tomorrow morning.... then tomorrow when she gets home from work, put down the trap, bait it, and leave it... she should check it if she hears it go off, or right before she goes to bed. That would minimize the amount of time the cat would actually be in the trap if it gets caught.
The cat is more prone to go into the trap if it feels safe - no people or other animals around - and also a little bit hungry. While I would never condone depriving an animal of food for a prolongued period of time, it usually won't hurt for them to skip a meal. (Domestic cats are really quite spoiled. If the cat were a stray, it could easily miss several meals in a row.)
Now, if she isn't in a rush to get the cat out of the basement, she could follow the same steps, but instead of setting up the trap, she could sit downstairs when she feeds the cat (bring a book or something). Don't have her sit right beside the food, but across the room. Let the cat eat, then remove the dish, and each meal time sit slightly closer to the dish until the cat is no longer afraid of her. Given time, the cat will likely come out of its shell on its own, but how long it takes varies from cat to cat. I've taken in foster cats who were completely fearless, and I've taken in feral cats that took years to trust people.