No, he already took the damage, and is not damaged again.
Still, according to the Wildshape Description, you heal hitpoints as if you rested 8 hours - does this also applay to subdual damage? (I know it does not apply to ability score damage and similar aspects) In this case, he would be immediately healed from any subdual damage that would normally still have to be regenerated.
As a side note - I am even uncertain if the Troll`s Regeneration could heal the subdual damage from the Tendricolous form - Regeneration does not work against all kinds of subdual damage (strenous activiy, as an example).
I think the best way to handle it (call it a house rule or whatever to your player, one that has been recommended by someone who plays a shifter in a powergaming campaign), is to rule that you can only heal damage of a form you are currently in.
(Remember, hp don`t change despite new ability scores, so for the purposes of the "body", your current hitpoint is your normal amount of hp)
Oh, and congratulate your player to this good choice for character and form. (I love shifters, and once I had much fun with the Tendricolous form - I nearly swallowed a dragon, if it was not for his high grapple check besides being smaller than me)
Mustrum Ridcully