Is 10,000 DPS too low for a heroic?
First of all, the biggest problem here isn't with your fiancée, it's with the jerks that are belittling her. That's the most important takeaway. People are literally the worst. Literally.
That said, yes, Tyler Is a Stupid Jerk and No One Likes Him -- 10,000 DPS is somewhat low for an Hour of Twilight heroic. But here's the thing: It's OK to do low DPS in a heroic, especially if your gear isn't perfect. That's exactly what heroics are there for, improving your gear and catching up to the rest of the class. And based on the armory link you gave me at the end of your email, she definitely has some room to improve in terms of gear. Her toon is holding a i346 staff and is without benefit of a single tier bonus. There's going to be a huge DPS disparity between someone with an i346 staff and someone with totally up-to-date gear from the Dragon Soul raid, tier included.
Now, Tyler Is a Stupid Jerk and No One Likes Him, I'll be the first person to warn you that gear isn't everything. With your average 5-man heroic participant pulling between 15,000 and 30,000 DPS, you may find that new gear only gets you up to the low end of the range. But that's OK too. The best way to become a better shadow priest is to practice.
You and your fiancée already have a solid grasp on the shadow priesting basics. Your email describes the shadow priest priority rather well: prioritize DoTs, Mind Blast when you can, use Shadow Word: Death when the target is under 25%, and fill in the blanks with Mind Flay. Don't forget to use your Shadowfiend when it's on cooldown and Dark Archangel, too. Deceptively easy, right?
Anyone who's been shadow priesting for a solid length of time will tell you it takes a lot of time to get a solid shadow priesting rhythm down. My DPS was embarrassingly poor the first time I ran a heroic, and it took months of raiding before I showed up in the top half of the DPS charts. Your fiancée will improve with time, too, so long as she doesn't let the jerkstores of the world get her down.
P.S. If you really want to cut off the bad mojo at the pass, I'd advise she start each heroic she's in with strangers by saying something along the lines of this: "Apologies in advance if my DPS is a little low, but I'm still gearing up this toon up." Maybe even throw in a little smiley face in there, too. She shouldn't have to do this, but I've found similar lines go a long way in disarming trolls.
Thanks for writing, Tyler Is a Stupid Jerk and No One Likes Him.
What kind of DPS should a shadow priest be expected do?
As I said, there should be a significant degree of leniency given to any shadow priest running heroics to gear up. The heroics that launched with Cataclysm are now the refuge of the just-turned-85 crowd, and as such, DPS numbers there tend to be low. And like we've said, that's OK. Once you start moving into endgame content, though, there are some certain numbers shadow priests should strive to hit.
Based on experience, most Raid Finder fights will have widely varying DPS numbers. For a stand-and-DPS type fight like Ultraxion, it's not unusual for the weakest DPS member to pull 12,000 to 14,000, while the best performer is up closer to 35,000. My moderately geared shadow priest was able to pull 28.5K there last night; take from that what you will.
Of course, different fights have different DPS ranges. The Madness of Deathwing fight, for instance, has a number of buffs available that can easily push a shadow priest's DPS up over 50,000. A fight with less action such as Spine of Deathwing, meanwhile, will see low numbers pretty much across the board. In my Spine attempt last night, the highest performing DPS only turned in 24.1K, and a handful of players were in the single digits.
Every player is different, and some folks may struggle to top the charts no matter how hard they try. And as someone who's been through a lot of raids, that's OK. Truth is, it's far more important in the Raid Finder for a shadow priest to DPS the right target than it is to do so with top-notch numbers.
Are you more interested in watching health bars go down than back up? We've got everything a shadow priest will need, from the basics of shadow priest PvP and advice on gearing up for Raid Finder raids to the hottest trinkets and weapons for today's shadow priests.