One thing the Wizard could do to help is buff the Rogue with Enlarge Person and perhaps Mage Armor or Bear's Endurance. That'll help a bit with the Rogue's melee survivability and damage-dealing.
The Cleric should cast Shield Other on the Rogue, so that the Cleric remotely absorbs some of the damage suffered by the Rogue. That way, the Cleric can focus a bit more on healing himself once in a while, rather than on constantly rushing into the fray to heal the Rogue, at least sometimes. And this only really requires the Cleric to waste 1 turn and 1 spell (though probably more than that if they fight several battles a day).
Personally though, I would suggest you or the Rogue's player gets the Tome of Battle: Book of Nine Swords.
Then he can multiclass 1 or more levels into Swordsage or Warblade, maybe multiclass into the Shadow Sun Ninja prestige class later on if he feels like it. Since he's already fairly well into his Rogue levels, this won't be a substantial power boost, but it will DEFINITELY help him out in melee, and even in his sneakiness.
Maneuvers use something similar to spell slots, but without level differentiations (a martial adept class just gets a certain number of maneuvers readied; using a maneuver removes it from their readied selection, until they do something to recover that maneuver, which depends on the class; maneuvers are recovered freely at the end of an encounter also). So a martial adept can use a handful of maneuvers in combat, but each can only be used once, until the character takes an appropriate action to recover that maneuver.
Swordsages can recover one maneuver with a full-round action (or maybe it was a standard action, I forget), while Warblades has to use a standard attack to recover all of their readied maneuvers (but they can't use maneuvers on that turn). Crusaders have a wierder mechanic for it, and aren't the kind of class your party's Rogue is likely to take. Stances work all the time, but a character can only have one Stance active at any given time, and needs to use a Swift action (same as casting a Quickened spell, so it can't be done in the same round as another Swift action) to change their active Stance. Only the highest-level characters of a particular class or two can use more than one Stance at a time. Martial adept classes learn a specific number of maneuvers and stances based on their level in the class.
Swordsages have access to the Shadow Hand discipline, so they can learn maneuvers and stances that make them stealthier and more deadly with Sneak Attacks (the Assassin's Stance, a 3rd-level stance, grants something like +1d6 or +2d6 to Sneak Attack). Shadow Hand includes maneuvers for short-range teleportation through shadows, and maneuvers for 1-round invisibility, or incorporeality I think (for one of the upper or middle-level maneuvers), as well as maneuvers that grant concealment (without giving the enemy concealment against the character), which would allow the Rogue to try moving somewhere to hide with a Hide check, in order to Sneak Attack on the next round or something (concealment is generally needed to attempt Hide checks, if not using a Bluff check to create a diversion to hide).
Besides the tricky stuff in Shadow Hand, there are some SH maneuvers that just boost damage, or improve your accuracy, or incur a bit of extra damage and some side-effect (like temporary Strength damage). SH even has one or two short ranged attacks, like Shadow Garrote (which is 2nd or 3rd level, I forget which). Also, regarding Shadow Hand, there's a cool feat available to people who learn a few bits of that discipline (I think it requires an SH stance and an SH maneuver, or somesuch), the Shadow Blade feat; this adds their Dexterity bonus to damage with particular weapons.
Other disciplines are available too, though Warblades have access to slightly different ones from Swordsages (the former have access to 4 or 5 disciplines, focused on physical might, mental focus, and leadership, while the latter have access to 6 of the 9 disciplines, excluding only Devoted Spirit, which is Crusader-only, Iron Heart, which is Warblade-only, and White Raven, which is exclusive to both Crusaders and Warblades). Most of the disciplines are focused on melee prowess of one kind or another; frex, Stone Dragon and Iron Heart are all about physical force and strength of will, whereas Setting Sun and Desert Wind are all about finesse and tricks (though DW is also about
fire...). The other disciplines are Diamond Mind and Tiger Claw, the former emphasizing mental focus and reflexes, the latter emphasizing savage fighting and limited offensive shapeshifting.
For a Swordsage, the Desert Wind and Stone Dragon disciplines are good for dealing out big damage, but Iron Heart is better for Warblades I think. Diamond Mind is also very potent for damage, but you have to spend a lot of skill ranks on Concentration to make any decent use of the Diamond Mind offensive techniques, which include Focused Strike (IIRC), Superior Focused Strike (IIRC), Sapphire Nightmare Blade, Ruby Nightmare Blade, and Diamond Nightmare Blade, and there's a few others that don't rely on Concentration (IIRC, I don't have the book on hand). Actually, right now I can't remember if the Nightmare Blade maneuvers all use a Concentration check or not, I forget. Stupid me. :\