I've noticed a difference between how players react when I run the games.
I have always been wearing kidgloves until recently and I noticed that when players realize that they have to strive for the things that they need, the challenge of it makes the game much more worth it.
I say take the kid gloves off. If this druid has repeatedly done this action, then don't be handling it with kid gloves.
you have several alternatives
1) He falls, suffers falling damage + the Wyvern/sorcerer coming to look for him.
2) He gets captured, fingers completely broken/cut off, can't exactly cast somatic gestures with no fingers you know. Gets interrogated, and depends on his action here, events unfold.
3) Have the Wyvern and Sorcerer have some fun, grab him by the legs and whack him across the tree tops repeatedly until he goes to 0 hp. After that, have them bring the druid to a Telepath for a mindwipe and have him get reintroduced to the players as an enemy.
4) OR The telepath hides instructions inside his head until the time is right for him to react. (It could be very well be killing the whole party when he cooks for them the next meal) your choice.