Yikes! That's tough. She doesn't poop anywhere else in the house???
I'd say, keep her off the couch
If that doesn't work, try moving her food bowl to the spot where she poops for a couple of weeks. Annoying, I know, but Kirby had it in for one corner of our dining room rug, after taking it out to be professionally cleaned, we sat his food bowl there for a couple of weeks. His little brain must've clicked and realized that this was an eating spot, not a peeing spot.
Good luck!