Using a large potted plant as a litterbox

I was talking with one of my friends last night. She is making an effort to transform her indoor/limited outdoor cats into more indoor only due to the disappearance of one of her cats early December.
One (or two) or two of her cats is using a potted plant as a litter box. She says it smells terrible. I advised her to take the pot in the garage and remove all the soil from the container and shake as much off the roots of the plant as possible. Then smell (i know it might me gross) the roots to see if they smell like cat urine. Replant with fresh potting soil and sprinkle red pepper flakes on the top of the soil.
Can she put anything else on top of the soil to deter the cats?? I was thinking crumpled aluminum foil??
Thanks!!

Yes, a whole bunch of marbles! Not so many that the weight will impact roots, but a layer would help. Also a citrus smelling spray made for cats - works on furniture too and shouldn't hurt the plants.

I use Cat Scat mats, they work FANTASTIC!

I use decorative stones in the plant pots, and on others I put plastic lids on them (you can buy them at home depot). This seems to work.

I use moth balls, but they do make the area around the plant smell funky...

Mothballs are very toxic! You should really consider a safer alternative for you and your pets.