Do you have Complete Arcane, Complete Adventurer, Complete Scoundrel, or Complete Mage? There's bound to be something good for that PC in one or more of those.
She could rely on invisibility spells and a shortbow for ranged sneak attacks.....taking Point Blank Shot, Rapid Shot, Precise Shot, Weapon Focus (she's going to have a poor BAB), and Maximize Spell or Empower Spell (for any Scorching Rays, Magic Missiles, Acid Arrows, Lightning Bolts, Shouts, Disintegrates, or whatever).
From the Tome of Battle, Martial Study (Cloak of Deception) could be handy, allowing her to go invisible on her turn once per encounter, which'd provide an opportunity for sneak attacks. Probably best used in combination with a shortbow for her, making multiple ranged sneak attacks in one turn. It'd also open the way for her to gain Martial Stance (Assassin's Stance) later, at 12th-level. That'll give her +2d6 more Sneak Attack damage once she gets it.
Rapid Reload could be taken at 9th-level, in order to use a light crossbow or hand crossbow at the full normal rate of attacks instead of 1/round, replacing her shortbow for more damage. Then Weapon Focus (light crossbow, or hand crossbow) could be taken at 12th or 15th level, and Crossbow Sniper (from Player's Handbook II) at 15th or 18th level, as appropriate, for more damage and more range with sneak attacks.
If she's interested in just getting good with a crossbow, she could take Rapid Reload at 6th, Weapon Focus at 9th, Crossbow Sniper at 12th, Point Blank Shot at 15th, and Rapid Shot at 18th. Of course, with the poor BAB of sorcerers and arcane tricksters, she'll need to turn invisible with spells in order to have any decent chance of hitting stuff. And try dispelling any True Seeing or See Invisible spells that enemies are using, of course.