At least he's a priest who understands that GH is better than Flash Heal. I've been noticing the same thing he has. Priests who think faster is better and not realizing there are far more important equasions to factor in, such as mana efficiency. For any newer priest who reads this post, here is a timetable to help you better understand the effectiveness of Greater Heal.
Greater Heal, talented (at 70) - 2.5 second cast time, 713 mana
Flash Heal, glyphed (cannot be talented, also at 70) - 1.5 second cast time, 424 mana
Let's say you are MT healing a boss. You have 10 seconds where you are spam healing him. Now, let's do a chart of sorts to show the differences between these two spells over that period of time.
Greater Heal - Over 10 seconds, you will cast this 4 times. It will cost you 2,852 mana, and your healing will be (estimated) around 24,000.
Flash Heal - You will cast this 6 times and be in the last .5 seconds of your 7th. It will cost you 2,544 mana and your healing will be (estimated) around 21,000.
If you take this exact same timetable and spread it out over a full raid boss encounter, you are doing more healing using Greather Heal than using Flash Heal. Notice how Greater Heal in 10 seconds of time cost only 308 more mana, yet has done 3k more healing. That's called efficiency. The HPS ratio on Greater Heal is far better, and it's mana spent to healing done ratio far exceeds Flash Heal.
In the end, Flash Heal as Holy sucks. It should never be your primary heal, and if you are using it as your primary heal then you need to look at this page and realize your mistakes. Math is not an opinion. It's a fact.
Kudos to the OP for trying to educate the priest community. Maybe now the bad healer population will decrease by some percentage points.