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

[NessaWyvern]

Also LOL at server blacklisting over LB trolling in a dungeon.... these people need to get a grip on reality.

The person was new (so I think they weren't trolling in the end. They died to a fair few boss mechanics also), and would probably do it, not just in dungeons, but any end-game content if they didn't learn about it.
Maybe it wouldn't be as drastic as a server BL, but I can see them being blacklisted from a lot of EX and Savage parties if they kept up with the "healer LB 1 when everyone is at max health, always".