While said player is an arse about it, he's got a point. As a DM, you really should hide your suprise at things you might not have fully researched unless you want him doing this more (and worse - feeling justified doing so).
I've had a rough run recently with a DM that threw (at a 5-Member, 5th Level Party) an ECL 15+ Encounter (3 L10 Casters, L10 Fighter and a Construct). And that was after a pair of encounters before that which were ECL 6 and ECL 8. We really didn't have a chance. I spoke afterwards with him (respectfully) that we're really not in a position to fight what he's throwing at us and survive. That he needs to scale things a little more reasonably if he wants to have a campaign and not 5 players who're going to start making pun-pun's to survive - cause it was getting close to that...
That antic would have gotten him a first and last warning that abandoning the party in that fashion. If he's going to get "bored" and make his PC leave the game, he may as well leave the group. Period.
However, for the sake of the rest of the party, scale your adventure when a PC does that. Otherwise it just stops being fun for everyone. As the DM, you could have potentially salvaged that with a little tweaking - and left Mr Druid sitting out twiddling his thumbs while the rest of the party had a great time.
In fact, that's exactly what I'd do. Since he left, he has no awareness of what's going on. I'd basically put the rest of the party through a power-levelling session just to spite him for his antics by setting him up to be 2-3 levels lower than the rest of the party...