In my experience of being a healing Druid (only did 10 mans, and having left the game for my studies for a little while) I can say that Druids do not excel at the jobs you stated, neither raid healing nor single target healing.
The way Lifebloom works, allows a Druid with good connection to stack it on 4 tanks without breaking it, 3 is easily possible without HEAVY lag, that means, 3-4 targets that takes damage the entire fight (not 3 tanks tanking 1 target but rotating the aggro). This is the best a Druid can put out because of the amazing efficiency of Lifebloom.
2 tanks are quite good as well for Druids, because you can put Rejuvenation into your rotation, though not as effective.
Many situations though, requires only 1 tank and raid healing, and these are the 2 jobs that people think of when you say a healer, Druids don't fit any of these perfectly though. On 1 tank you just can't put more HoTs for more HPS, you are stuck with alot of free time in your rotation (with the exception of Swiftmend, you are casting 4 Lifeblooms, 2 Rejuvenations and 1 Regrowth every ~24 seconds, meaning you only spend 11 seconds of the time for casting, up to 14 if you lag abit, that's about half of the time only), this is frustrating, knowing you have more time and more mana to put out, but you are not needed, even moreso when others have to work higher. Be sure that healing meters won't help here, no one wants to be left behind on the meters.
On the other hand, HoTs are very MEHish to raid healing, as no other healer will wait for the HoT to tick. It was hard for 1-2 healers to wait for my HoTs in 10 mans, I can only assume how hard it will be on 25. Be sure, though, that he is NOT a better raid healer than Shamans and Cricle of Healing Priests.
It's hard for me to judge which is better in that case. I assume it's best to assign him on what's lacking, but combining the two is also good, keeping Lifebloom stacked on the tank and tossing a raid heal here and there.
I'm sure others will share their experience of 25 mans and be of more help, take from my experience what you will, while remembering I never healed a 25 man raid.
Hope I helped somehow, and good luck,
Yuval.