Raymond Chen has written about this. He doesn't even touch on what I usually think of as the primary reason that a threaded control wouldn't work well, which is that a thread shouldn't draw on a window associated with a different thread.
I encourage your co-worker to revisit whatever it was that made him decide that he couldn't put the task into a separate thread. It's simply not a good idea to block the main UI thread, regardless of whether there's an animation control there to cover up the non-responsiveness.
