Meter padding is when you're LOSING useful dps to do ghost dps. Like rogues bladeflurrying council in toes. Yes, that's crap and useless. There's nothing useless about bringing bosses HP down, like rogue bladeflurrying on council in ToT. That's what he's best at, this kind of cleave, why would you have him in if not for that on this fight?
Anyway, back to the topic at hand. I'm assuming from armory this is normal mode. How we do normal is we bunch all bosses up, /care about the healing adds, cleave the hell out of them and profit. I'm healing those runs and I always HOPE we get sul empowered because there's nothing to heal otherwise... We do this on our shitty alts too, so maybe that's an idea. Honestly, the panic about sul is a bit over the top. He's really not that scary and it will take a lot of stress off your raid.
As for your lock's dps. There can be so much wrong. No multidotting would be one thing - and by the way, he should be draining/shadowburning the add at least for shards/embers by the way. But just playing random, not paying attention to procs, that's why most shitty warlocks are shitty. That's not something you fix overnight. That said, gearlevel doesn't say much. If his trinkets and weapons are crap, or has no setbonuses, he will be doing shit dps even if he has heroic shoulders, you know. Regardless, he should be doing more than he's doing. It's kinda hard to fix from here without seeing him play or anything, but lets give some general pointers:
- Rain of Fire; make sure he keeps this up on as many targets at the same time, but even if there's only one it should always be up. He needs to find a way to track RoF, as there's no UI function for this in default UI. So if there's two targets bunched up even if Sul is somewhere in Tokyo, he puts Rof on those two targets, to maximize ember generation.
- ABC. Always Be Casting, probably the biggest reason why casters have issues with dps (and melee too, really, think of those rets who take coffee breaks between their crusader strike and their exorcism proc): most often this manifests when you change spells: incinerate spam goes fine, then suddenly he has to switch and cast chaos bolt and by the time he found his button a second came by and went (this is not an issue exclusive to clickers by the way). Do that the entire fight and your dps will be crap.
- Fire and brimstone. If bosses are hugged up as they should, except if you really wanna do it properly the priestess, fire and brimstone at least those immolates (though yes I'd even say fire and brimstone a lot more than just that... :P). It gives shitloads of embers, and he can just spam chaos bolts away on Sul. Havoc another boss and incinerate away for more embers (or, gasp, chaos bolt to khazra for example, but maybe that's meter padding too... ). He must always maximize the ember generation for maximum (singletarget) dps.
- And lastly, use those chaos bolts only when he has a sexy proc or when he's about to cap embers. He has two half decent trinkets, make sure those bolts are at least aligned with those or with dark soul. That said, let him get Jade Spirit. Windsong is so incredibly crap compared to that. It's expensive but surely your guild can help out? Same with bracer enchants. Getting your gear in order is crucial to up any dps.
I really didn't mean to sound harsh to anyone, I went back and corrected a lot in my post I hope, I'm sorry, but it always gets me a bit iffy when people complain about meter padding when honestly there's nothing meter padding about that. And anyway a lot of dps are in fact also bad because they just don't do their max, they focus so hard on 1 job they completely forget the rest and that's how they die of fire cause "didn't see it" - or, when an add spawns even if there's a skull somewhere to SEE it and realize getting dots up on it will be greatly beneficial to both your dps as well as the raid, or just realize that a shadowfury right now on that other add would be an amazing contribution to the entire raid too. Just... you know. loosen up a bit, take some time to take in your surroundings and what's happening. I think that would greatly improve many players.