Tactful ways to inform people that they're playing badly?

[Granyala]

I just type what they’re supposed to do

This.
I present the data in a take it or leave it fashion.

Though I have to be honest here: it's very rare that I encounter a random that makes such critical whopper of mistakes as the OP describes.
Most of the time I'm too busy playing my own role in order to notice anyway.

There was a WHM in my lvl 79 dungeon, who kept using the LB as soon as the bar filled, even if everyone was at full health. I said "I don't want to be that guy, but the LB in dungeons is usually reserved for Dps " but I got no response (despite him talking earlier in the dungeon) and he kept doing it. He uses healer LB at one bar again a few more times with everyone at full HP, and since I was starting to get a little annoyed, I asked them "So, uh, is there any point in using healer LB when everyone is at full health?" still no response. Might not be the most tactful way of wording it though,but I assume it sounds better than "Stop wasting the LB, healer."

I wouldn't usually say anything, but this was a lvl 79 dungeon, and he's gonna get a lot of hate later from others if he keeps using healer LB as soon as the bar fills.

Haha.. people still care about that dumb limit in dungeons?
Unless you AoE trashpacks, I'm pretty sure it's a DPS loss anyway. 1 bar LB is so useless and stuff should die fast enough for there to be no 2 bar available.