wow, I amost don't know where to begin. But/so this may ramble a bit -
First, I think that properly specced & geared healers are ALMOST all equivalent, both in healing & damage. A Holy Priest, Resto Druid, healing shaman, and healing pally don't have so many differences, if all/most of their talent points ar in the healing tree.
That being said, on to your problem: the group setting.
I don't know how you all were set up (I don't know the other 2 classes and roles), but I would GENERALLY put the Priest on the main tank (and possibly the OT, if there is one), and I would GENERALLY assign the Resto druid to a balanced role of DoT/HoT/debuff on the rest of the party.
If the person is PRIMARILY resto specced, at that level, he/she probably has a backup set of feral gear (cat, bear, or in between), but very probably few or no points in the tree, and little experience in a feral role.
Sometimes, like with the Priestess in Blackrock, you will actually PREFER a druid to tank (because of being immune to polymorph), but generally, a hybrid class should preform their primary function, and scatter in secondary stuff when possible.
Taking your situation above, I would have had the Priest on the MT (and OT, as well, if the person feels comfortable and capable, and the OT is a class that can't really self-heal), and the druid alternating between whatever roles are needed - heals for the other party, crowd control, debuffs, HoT dps, etc.
I've mentioned before, I'm Balance-Resto, but I am a pretty powerful BACKUP healer. You really don't want me as the main healer, but I can & will heal effectively for boss fights, or pop out of Doomkin, barkskin, and throw out a tranquility or combat rez that can otherwise save a wipe.
As both Das and Tag said, I think it is VERY important to define roles up frot, and also discuss strat before any boss fight.
~Darq~