Cynfael,
If he was just standing there for like a minute, and just threw one or two heal, then you can fairly tell him : "Hey friend, since damage so low, why don't you try throwing in a few stones or aeros ?"
Then may be develop : "More damage will make the mobs die faster, so in return it'll make you heal less..."
And go on like this in a courteous and constructive way.
That is perfectly fine.

I don't know if it's a new trend among youngsters nowadays, but the problem is that people will most of the time yell on pure healers because :
- that make them look like pro gaming masters
- there's that stupid "linux-like" habit that hackers have of yelling, because according to their guru, dissaproval is better "understood" this way
- as you said, some pro-DPS healer tend to put DPS before healing, which is WRONG
- anonymity just transform innocent kids into the worst form of troll that our world can produce

In fact, it's not a problem about gameplay at all : it's a problem about attitudes.
And from my experience, in most group where I had those "elite" players yell on noobs after many wipe, once the "elite" finally ragequited and we got replacement, dungeon/raid surprisingly ran smoother to completion.