Honestly? He's probably lying to you. WoL isn't worthless. The raid composition doesn't matter at all. Well, a beneficial composition can make a difference of 1-2% but that's negligible. The spec doesn't matter either. Holy priests usually have the same spec with only 2-3 points deviating from each other's spec which are personal preference. It doesn't affect throughput at all. The "role" of course has to be factored in but either your healing the raid or the tank, you can easily distinguish between these two in WoL. Again, he's probably trying to hide that he's not really a very good player.
Holy Word: Serenity (instant heal, easily more than 10k, has a CD though).
Heal (if there's enough time to cast it before the next Caustic Slime)
Flash Heal (in case there's not)
Binding Heal (whenever I'm one of the two targets)
This is for the normal phase. In feud phases it's:
Prayer of Healing (spam it, yay
)
Circle of Healing (whenever available)
Lightwell (click it please!)
Holy Word: Sanctuary (just once, put it down at the beginning of the phase, you don't have to use it though)
The thing to keep in mind that there are only three Caustic Slime attacks in that phase followed by a Massacre. This means you don't have to heal everyone to full. Just enough to survive 3x Caustic Slime and still be over 10k as the Massacre will kill all the other full hp bars you may have created ;-)