My only problem with taking the drain apart is that it is a pedestal sink that is pretty tight with the wall, and I would probably have to break the sink itself open. I think I'll try Draino Foam.
After discovering her "trick" I check all the other drains in her bathroom and mine just in case. She does swear that this one was the "only" one. I guess I'll just have to keep an eye on the others.
It is actually a very funny thing to have happened only because the baby sitter discovered it with her, not us... ah the memories!