Honestly, since you're still in normals, it doesn't sound like your guild is of the cutting edge kind and therefore that the players in it aren't of the cutting edge kind either. In my experience, with such guilds, (which I have been a part of on my alt because I sincerely do enjoy the different mindset from my main's hardcore raiding, so no hate or anything), you should be happy that he does any kind of DPS at all, no matter the spec.
People are grossly overstating how equally well destro is to affliction here, by the way, and just ignore the sims for a second. The main problem destro has, is its very low skillcap as opposed to affliction, where affliction can pull such heavy numbers if you're really good. There's no beating a good (geared) aff lock even after the nerf by any class, honestly. By geared I mean mostly trinkets, which make a huge difference. That said, multidotting is so incredibly strong at the moment (protectors (if that's what they're called, the softfoot guy and company), general nazgrim), that Destro's aoe is really of no importance and a lot of classes do a better job on the few fights which do require constant aoe (heroic shamans, the guy that flies in the air with the towers and shit (I'm so good with names
)).
Archimonde's Darkness is a very solid talent. As Affliction, I have found only 2 or 3 fights to favor KJC, and I honestly don't see why this would be different for Destro (but meh, I may be wrong).
Nonetheless, if he's barely above your 120k tanks, his dps is low. That sounds more like a player issue than a spec issue in short, and as someone pointed out, if you're not very good at aff, destro will be equally good if not better.