Most heals in 4e were minor actions, or part of an attack, so chances are he would have been able to try to do that and still do the heal.The real question is whether Grorek could have destroyed the zombie in one hit at that point. Then an order to do so would have saved the dwarf.
Yeah not sure offhand what class he was, but just about any 3rd level character is going to have a minimum of 30hp. Even a Wizard with 12 con (basically minimum for a dwarf) is going to have 12+10+4+4 = 30 hp. A tougher class/someone with more con can be as high as 45 hp.It also says "4th Edition extended the death threshold to a negative number equal to half the character’s maximum hit points." so I guess the dwarf would probably have survived getting reduced to -10 or lower. A third level dwarven warrior ought to have more than 20 hit points total.