Interesting catch. I see what's going on. The woman's right leg is bent so we see her right foot behind her. But Elkin draws her skirt billowing out in a straight line from her right thigh, giving the impression that the right leg is extended, not bent, and it's holding up the skirt. The "stump" is actually the corner of the couch, but unfortunately it's right in line with the fold on the skirt so it looks like part of the extended leg. It would have worked better had Elkin drawn the skirt draping downward from the right knee rather than billowing out. For what it's worth, the couch is out of perspective, too, but that's probably nit-picking.
Link to the book:
Dear Lonely Hearts 3