According to my math over at http://www.healadinne.com/HealCalc/
HS is about as expensive in terms of HpM as GHW (assuming no 2set), and has much higher HpS, so it would be a good substitute for that. It is however considerably more expensive than HW. I may be wrong. I can't see it as a "filler" however, as in something to cast when there's very little damage going around. In terms of Mana per Second it is the second most expensive spell, right behind Healing Rain.
It also benefits from crit much less than the other single target heals.
Them's my 2c, hoping some experienced shammies will correct me.
Is he not running out of mana and/or hugely overhealing with HS as a filler?