See, you need to tell them to stay the fuck back. If they want to attack, pull mobs back to them, instead of them moving closer to the mobs.
Mobs have an aggro range that is level dependant.
Basically, from level 1 up to some level below them (maybe 10 levels below the mob?), the range is pretty much static there. From there, the range decreases the higher the level you are. After some level difference where you're higher level than them, you pretty much have to brush against them to make them attack you.
Now, if you are level 37, the mobs are 32, and your druid is also 32....The idiot druid has a higher aggro range than you. Ergo, he stands -behind- you, not rush in.
If they're a ranged class, usually it's not a problem for them to start attacking along with you. They're not gonna pull more mobs because they're already at distance. At worst, they'll pull a mob off you by causing more threat, but one mob is a whole different monster than five to eight mobs.
If they're melee though (and from the sounds of it, he was a feral druid), them moving closer is a very bad idea usually. Chances are that what's a safe distance for you probably isn't for him, and a lot of people measure their safe distance by their party members.
I've been guilty of this when I've been escorted by a level 70 to places and then going "Oh CRAP!" when I invariably pull something, admittedly.
Still, the best thing to avoid deaths when partying with someone of lower level are to set some ground rules.
1) They stay behind you. The bigger the level difference between you, him, and mobs, the more distance to be put.
2) They do -NOT- pull. You do. If they're melee and they want to attack, they wait for you to pull mob back.
3) Unless you're overkill for the area, use raid symbols (you can set them by right clicking on a portrait) to set kill order when you're pulling a group. If they're hitting the same thing you are, there's less of a chance of them pulling aggro away from you.
4) This one's more for you. If the mob fears, pull it as far back as possible. Even if you're less susceptible to fear because of your higher level, your partner is much the opposite. Chances are he'll get feared into the next mob, they'll rape him, and then move in on you.
If rules are broken, give them a warning and call them an idiot. If they do it a second time, break off the party. Or, if you're a class that can fuck with them, pull a large mob group to them, push aggro onto them and flee/feign death/bubble and watch them curse you. Then ask them how they like them apples.