As the healing lead, your guild leader gave you full discretion with your healers. But you need to move fast. Healers always face an enormous amount of pressure, and you can't always afford to take your time and wait. Once the weak player is identified (and you've figured out that it is
him causing all the wipes), you have to move fast and sit him. If you want to give him an additional attempt or two at a different assignment, that's up to you. Have a plan in place if he's not able to pull it off. If he succeeds, then it's great news because you don't need to interrupt your raid. If he doesn't, then the time has come for him to tag out.
With luck, you have a healer on standby doing dailies or engaging in some PVP of some sort.
You whisper the healer and say that you need him to step out right now. If he's perceptive, he'll nod and simply hearth without giving you any trouble. He'll understand that he's the weakest link in the chain and that the raid is in full progression mode. That healer is clearly not firing on all cylinders and you want to pull in someone who can, because the rest of the healers just aren't at the capacity to carry him.
You whisper the healer and tell him that you need him to step out right now. He asks why and proceeds to throw a tantrum. This is actually the easier of the two because if he blows up, you may as well have a reason to punt him out of the guild. But if a healer generally gives you some kind of resistance, you can tell him you'll explain it later after the raid.
Whatever happens, it falls to you to do your part to keep the raid moving. Sometimes this means that players need to be left in the dark for a while. Let him know that you'll explain it to him later. With luck, he'll understand. If he continues to press, you may have to take a few minutes and stop the raid temporarily to explain the problem. Depending on the guild atmosphere, you could consider rattling off the reasons verbally in public. Hopefully, it won't come to them.
When you get the substitute healer in, you have to bring him up to speed fast. If he was diligent, he'll have an understanding of what's going on in the encounter. Just explain what you need him to do (healing assignments, cooldowns, and things to watch out for). Keep an eye on what happens here. If the new healer who is brought in experiences similar difficulties as the one who was asked to step out, then the problem might not be individual performance. It could be the actual healing strategy used. You won't find out until the end of the raid or until the boss has been downed.
So now that the raid has ended, what is the next step?
Simple! Track down the healer who was sat and have a conversation with him. List exactly what their errors are, if he hasn't realized it yet. In order to fix a problem, it needs to be known first. Surprisingly, raiders just might not know what they did wrong (or to be more precise, what they didn't do right).
If he falling through spiderweb holes, tell him that he needs to cut his camera a little closer so he can see better or that he needs to work on moving more quickly.
Getting destroyed by assorted fires? Remind him of his self-heals, spells or that he needs to move more quickly.
Can't seem to keep up the designated tanks? That one requires a little closer inspection. If you managed to get that progression boss down, you'll want to do some comparisons between the healer who got sat and the healer who was able to keep the designated tanks alive to get an idea of what they were doing differently. Who knows? It could simply be an issue of gear. If it is, that's an easy fix. Granted, that might take a bit of time (or gold). At least you know what the problem is.
If the question is on player skill, you may need to tread a little delicately here. I suggest not using the word "you." Instead, approach it from the perspective of what is happening. Refer to the actual healing or the damage.
Example The healing that's being cast right now just isn't enough to offset the damage that's coming in. It either needs to hit harder or hit faster. Can we come up with something that can help, or is there something that's slowing down that healing?
Be sure to hear out your healer.
Sometimes it ends up being a simple case of raiders being too far out from where they need to be. I had a rogue who would constantly keep dying on Beth'tilac. I questioned the healers as to why that one player kept on dying repeatedly on attempts when there wasn't anything seriously fatal going on. One of my healers responded by stating that he was completely overextending and out of position since the rogue was going after spiderlings right away. I could either move a healer out closer toward the rogue, or I could ask the rogue to not overextend as far. Given the two choices, I figured the rogue was better off staying closer to the group so that he would be in range of every other healer and could assist with other spider groups in the event he blew up his early.
In another case, I discovered that the healer found that she was simply moving too much trying to avoid the different spells and abilities that were used in the encounter that cut down on her ability to heal. After thinking it over thoroughly, I ended up pointing out certain safe spots to stand in and to position herself in such a way to minimize the amount of moving that needed to be done. Sometimes it couldn't be helped, because she was constantly targeted by void zones or rockets of some sort. Other times, she simply moved enough to get beyond the edge of the damaging visual spell instead of bolting to the other side of the room.
There's nothing wrong with sitting healers. Just be clear and direct with them on why they got sat and what they can do in the future to ensure that it doesn't happen again.
Need advice on working with the healers in your guild? Raid Rx has you covered. Send your questions about raid healing to firstname.lastname@example.org. For less healer-centric raiding advice, visit Ready Check for advanced tactics and advice for the endgame raider.