Was doing SFK on my druid, with my friend's druid (me balance healer, him feral tank), we had a warrior DPS/off-tanking and a warlock with us. There was a 5th but he dropped real fast and we were too lazy to LFM.
Anyway, the three non-warlocks in the group kept asking the other lock to pull out his imp. We said it in party chat, whispered him, even put up raid warnings. We had 2 tanks already, and the imp would have really helped us out. We started paying attention to his playstyle, and noticed he spent half his time meleeing!
At this point I just said (in party chat, too, so he could see it) if him or his pet gets aggro, dont pull threat off him, and I wasnt gonna heal or buff him. Finally when we're 2/3 the way through the instance, (after his pet finally dies) he whispers me and says "my other pet's weak." I had to explain to him (and so did the rest of the group, which we'd already done about fifteen-fold) that the health aura and the extra DPS would help us out a lot more than an extra tank.
Well, he DC'd a few minutes after that but the run actually went smoother until we got to the last boss (where the MC and the crits from void bolt killed us real quick). But that is probably the first warlock I've watched play and had to tell him to reroll a rogue.