With the way mana is right now mastery is generally favoured as it gives a bigger return on the amount of healing done vs the amount of mana used. Haste will give you higher heals per second generally but will lead to greater issues with mana if you are not careful about managing mana effectively. What sort of raid progression have you got out of curiosity? If he is not having issues with mana management and it works for his playstyle then Haste is ok though mastery does have a slightly higher value.
http://www.mmo-champion.com/threads/...adin-Guide-5-0 this guide gives a nice break down in section 5 on stat weighting. Haste is just behind mastery but not by much and there are a lot of varying factors in the stat values, such as raid buffs.
I often find myself thinking "zomg cast faster dammit!" when aoe healing or trying to catch someone on low hp before they die. I have a primarily mastery build.