From what you said it sounds like it snapped. When the wing loading on a Sundowner is high it has very ugly stall characteristics. Instead of a gentle stall (like I got on my review plane) with light wing loading, the stall for a high wing load is a pretty severe snap into a death spiral like you described. These usually happen on approaches because the pilot tries to slow the plane down too much and "float" it in, instead of landing with a bit of power to avoid the snap/stall. In your case, he slowed on a dead stick and snapped/stalled at altitude.

Ken