Put a pan where he uses it to poop. They get a cycle going and hard to break it. Cats are born knowing how to use litter boxes. I had one that wud not use a box so I kept covering the spot he wud use with a rock so he cudnt get to that SPOT. It worked. Either put something in its way so it cant get back to same area r give it a pan behind the couch. I gradually mocwe it to whr i want the pan after i see they got the idea good luck!
Could it be stress related? Is she in a new place or has something changed recently? Also, is the litter box being cleaned regularly? Cats can be very finicky about cleanliness and some won't use the litter box if it's too dirty.
I would suggest to click on Pets above and go on the Expert Forum and do some reading, perhaps you'll come upon a similar post that a Vet has replied too....theres good reading in those old posts...
But just in my opinion from similar circumstances with my cat(s)...when they do something inappropriate like that peeing or pooping outside a litter box its because they are trying to tell you something....like I'm hurting.....can she notice any sign of blood in the poop? Is it real hard and constipated or loose?
The site needs to be cleaned with cleansers that will remove the scent or the kitty will think its ok to keep using the same spots.
do some reading in that Vet forum, I'm sure they'll have some good reply's....it may well be worth a good check up too...
good luck and let us know what you find out ok.