Christmas Tree

First, have your parents tried spraying Feliaway around/under the tree where you typically pee? It's got "happy-cat" pheromones and can prevent accidents outside the box. It helps some cats more than others, but it would be worth a try! Second, putting carpet runner pointy-side up could prevent you from walking under the tree (ugly though). Finally, there's also the "scat-mat" which puts out a mild electric shock (like when someone shuffles across the room and touch something) when you walk under the tree. Less pleasant, more expensive, but effective.

Is the tree in the same spot? Ensure you get all of the odor out of the carpet.

Also, is it a real tree? Perhaps there are other smells on the tree. My cat was afraid of tinfoil so I put that under the tree for awhile to discourage laying / bathroom behavior under it.

Thanks for the ideas. I'll have to let them know.

As far as the tree goes they have had both a real and artificial tree and I've peed under both. They'll be using the artificial tree this year.