Oh you poor thing! I would first spray the couch with a homemade mixture made out of the liquid you put in a septic tank. The stuff is made from enzymes. Dilute it in a spray bottle at about 1/15 liquid and the rest water.
The stuff literally digests waste in the septic system so it will eat anything natural. I couldn't believe when I saw a friend clean up filthy ugly carpet with who knows what stains all over it. I think it's genius that he thought to use septic enzymes on it. It worked incredibly well on his cat pee smell issue. Like magic.
I don't know if it will damage your leather, so if you use this, test it first. Leather, is also natural.... The stuff will, however, get out any tough stain on the carpet, no matter what it is. Especially pee and poo. And, it is cheaper than stain cleaner of any type.
Now that she's peed in the box, it probably won't happen anymore. Just a guess. She just didn't know what to do and the leather seemed like the most natural thing to her. It makes sense if you think about it. But sorry about your sofa~