on
slime boss he did less than half of healing done by the second healer.
* he had time to cast 16 flash heals but 0 penances and 0 poms
* almost 40% overhealing with da, almost 60% overhealing with poh - means he spams poh blindly to stack da - and nothing wrong with it if the da is consumed. but 40% overhealing with da? means he spams poh to build up high da and then doesnt refresh it, lets da drop - can you imagine how much healing/absorbs are lost there - no wonder he has mana issues.
now
hagara
* again 0 penances
* flash heal spam - flash heal isnt any good for a discipline priest, who does have mana problems.
conclusions:
* tell him about penance spell and pom spell
* ask him to consider using penance instead of (at least some) flash heals
* maybe suggest him dropping atonement spec for a week or two and use sos spec instead - he will loose smite/hf buttons and will be forced to rely more on healing tools. after that he can of course pick atonement spec again.