It all depends, if it is a single mob, and the DPS is pouring out crazy damage, he can prolly handle it. toss him 1 heal it's prolly all he needs.
But if he pulls a group of 5 before the tank is there. let him die, and run behind the tank. no need for you to get proximity aggro once the DPS dies, let the tank grab everything and continue the pull as normal.
Also, in my experience for example at level 19. i bring my twink into randoms for fun. i have 1500hp's on an enhancement shaman and 300ap. a tank can not pull aggro off me. and i have 3x as many hp's. this really can't be helped unless i strip down to just chest and helm lol. i've had healers and tanks complain that it's not my job to tank. yet once i let the tank take over it becomes a wipe fest due to his 400 hp's and crap threat.
Moral of story, take it all on a case to case basis.