Wet exits are the fallback and aren't hard. Just pull the strap on the spray skirt or, failing that, grab a double-fistful of skirt and pop it off the side.
Probably the instructor knew about that, and something went wrong anyway? I wonder, then, what that might have been
Sometimes people tuck the grab-loop inside their spray skirt by mistake, which is why it's worth practicing one of the back-up techniques. It's also possible for an improperly burped dry-suit to trap someone in a boat (I know of at least one fatality in this way). I wouldn't expect an instructor to make these mistakes, but it's possible.
Oh, that sounds really dangerous to forget / happen