@Chloe Sullivan : If you want a single thing from outside your usual disciplines, you could just grabt it via Advanced Study. Since Maneuvers count towards their own prerequisites, that'd cost you two feats - one to grab one Eternal Guardian Maneuver (requiring zero or one maneuvers as a prerequisite), the other to grab the Stance. Well, technically that'd require retraining (so that you first meet the prerequisite), but that's just a matter of gold (and thus irrelevant for an NPC).
But to answer the orginal question:
There's seven disciplines that work well with ranged attacks because their strikes can be applied to them: Elemental Flux, Riven Hourglass, Silver Crane, Sleeping Goddess, Solar Wind, Tempest Gale and Veiled Moon.
However, out of those I'd call Silver Crane, Solar Wind and Tempest Gale the main ranged ones. Elemental Flux and Sleeping Goddess only work well if you have animus and/or power points anyway.
Silver Crane has bows as a discipline weapon (which is why I sorted it here), but actually has a bunch of maneuvers that are melee-only. It offers a bunch of movement without AoOs, which is good for archery, and otherwise lots of healing, which is very nice to have if you want it.
Solar Wind adds a lot of fire-damage, which can have it's drawbacks. It has a boost to negate wind-penalties, one to increase your range (very useful for firearms, even better for scatter-weapons), and several debilitating effects that work off saves or combat maneuvers.
Tempest Gale has a lot of maneuvers based around using Sleight of Hand for ranged combat maneuvers. For negating Wind, it has a stance. It also has a 5th-level maneuver that forces flying enemies to land, which is very nice.
If you consider dropping either Solar Wind or Tempest Gale - I'd honestly rate them pretty much equal, but for Firearm-users I'd recommend Solar Wind over Tempest Gale just for that one range-increasing maneuver. Tempest Gale is great if you want lots of ranged combat maneuvers, but Solar Wind offers a bit there too and if you have a full-BAB build, you have less need for using a skill instead of your attack bonus for combat maneuvers.