Your friend just needs more time in the saddle. As others have mentioned, MB the moment it is off CD is essential. Most fights don't have constant single-target up-time making a lot of the "basic DPS mechanics" scrutiny in WoL difficult (multi-add dots hide SWP uptimes, bloat DI procs, etc. unless you get into the nitty gritty of specific target uptimes).

Let's look at the normal Jin'rokh kill on June 19. Jin'rokh is the only single-target and easy to examine full dot uptime fight in ToT. All others have mechanics of some kind that will skew the numbers one way or another. Zandrax took DI and got 8 procs from it yet only dished out 17 mind blasts. He should have theoretically been able to get 19 or 20 MB off without a single DI proc so that's the first thing I would examine. I recommend making a Weakaura for when MB is usable AND something to track MB's CD so he can plan movement and other actions so that he can consistently and immediately cast MB when available. I'd give the Weakaura a really obvious (loud, short and clear) sound so that, even if his attention is completely focused on, say, running a Focused Lightning away, he'll have that audio cue to know he needs to MB ASAP.

His SWP uptime on this fight was about 96%. Not too shabby. VT was a little low at 89%, but nothing terrible. He can get that uptime of both dots closer to 98% by planning his movement--Need to SWP refresh? Use that global casting SWP to move toward the edge of the pool so you have less distance to run for Focused Lightning balls or the Storm phase. Be sure he's Mind Flaying at least 3 times (recast it and clip it repeatedly to stack the speed buff) and use Body & Soul or a feather to burst-run for heavy movement situations (tank screws up being thrown opposite the raid, you've just dropped off a Lightning Ball and the storm is coming, etc.). On that fight, you can also tell him to find the edge of the crater left by the tank so that he doesn't get stunned but can get right into the pool of water with a quick side-step. Also note that this is a DEbuff so the damage increase gets applied dynamically as soon as you're in it.

I also recommend Power Infusion for Jin'rokh instead of DI (although DI is very strong, the pool damage buff stacks nicely with PI) and wait to use it until the instant you step inside the first pool. Line up any on-use trinkets and Shadowfiend for this moment, too. The raid should also be Bloodlusting-Hero-Timewarping at this time so stack as many buffs as possible and then be sure to refresh both dots again just before the collection of buffs falls off.

And just a heads up, the warlock is far more the under-performer in that raid then your shadow priest friend and the RL should hopefully be even-handed with criticism.