Just in case you didn't know....yes....one can lose their dentures in freefall.
It's normal/necessary practice to ask your tandem passenger to leave their possessions on the ground if you think it's possible for he/she to lose them in freefall. Dentures are one of those things that we never really thought about. In the many years that we have done tandems, we've never ran across this before. We had a passenger this last weekend lose her dentures (upper jaw) during the fall. We couldn't detect when (you couldn't tell in the video). If they refuse to take them out...I'd instruct them to keep their mouth closed the entire time. Otherwise...it could be a really expensive skydive.
-Cory