Did you die? If not, he has done everything right. DD's normally just get a medica II or regen...
I'd say this is wrong ^^. Most heal-only healer are lacking "healburst" in extreme situation and are normally bad at healing (wiping city, dun scaith etc.) or if tanks drop to fast and vita is not enough...
Normally I let tanks drop hard, but as long as they live, who cares?
With 10% hp you are still alive and can do whatever you want to. It ends with 0, not 10 or 50%.
We have instant healing for reasons