I don't know if Rune of Mending is more efficient if you cast it on yourself, but I'll assume it's not.
He gave Ardon +7 hit points. He was in single-digit, so his maximum hit point amount was 9. 9+7=16. The crit inflicting -26 points of damage, so in the best case, the dwarf would have been reduced to -10 health anyway.
I also don't know if 4e has changed that, but in 3e at least, -10 is the "really dead" threshold for everything except the Tarrasque. If it's the same here, the +7 hit points would have been wasted since he was going to die anyway; and healing Ardon was better overall.
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.
Looking for "D&D 4th edition death", I found this
which says: "In 4th Edition, raise dead became a ritual that required 500 gp to cast. In addition, the target suffered a –1 penalty to most d20 rolls for six encounters of adventuring." The dragon's hoard looks like it contains more than 2000 gp, so resurrecting the whole party doesn't seem impossible.
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.