It appears that when using combat prayer for a group fighting Vorenor Winterbourne, the healer also inadvertently heals the boss at the same time. While this probably only adds a few seconds to the fight, it definitely seems like this would be unintended behavior for the combat prayer ability since enemy NPCs, especially bosses, should not be directly healed by players.
Additionally it appears that other buffs can be applied by healing, such as Spell Power Cure, as well as the range of buffs normally applied by combat prayer. Igneous weapons is also being granted to Winterbourne, appearing to give him a significant boost to his power against players.
Healing given to the boss:All buffs granted to the boss: