Hiding, even in plain sight, is a skill check that requires a.. erm.. is it standard or move equivalent... a "non-free" action. That means doing it in combat isn't terribly unbalancing as it's taking up precious rounds. Remaining hidden and unnoticed slows down movement (without -20 or so to the check), so the tiefling rogue ranger shadow dancer will be close to stabbity range enough to make it very dangerous for him on a regular basis.
Dual wielding rogues are hardly a new thing, they balance the massive damage output with the knowledge that if they don't kill whatever they've thoroughly annoyed with Sneak Attacks, it'll pound them. A tiefling character, with 1 hit die below the group norm (due to ECL), is going to suffer from this even more than a regular ranger rogue. Also, unless one of those weapons he uses is a defending weapon, he's lowering his defense without a buckler (barring use of feats that restrict twfers to fighting with one hand in exchange for ac).. so he's even more of a pickle if he doesn't auto kill his targets.
Two weapon fighting (erm Ambidex now I suppose) also causes a penalty to hit, something that a ranger can deal with much better than a ranger / rogue... Meaning the guy will be risking low defense in exchange for hitting less but doing more damage, and getting more attention, when he does so.
On the whole, this tiefling ranger / rogue doesn't need that much to keep him in line with the rest of the group powerwise.. Just things that hit hard, and don't insta die from a sneak attack in the surprise round and the possible extra 2-3 in the first round. On paper, and against average foes, he might be a killing machine, but the first time he encounters serious resistance, and is possibly pasted, the player's mindset will shift in a manner that will keep him from ninja-flip-out-blendering into every combat.
oh, and I'm surprised he invested two feats to get wings.. that couldn't have helped him powerwise (again, more reinforcement for thinking he's a-ok as is.) and secondly, aren't rangers getting Hide in Plain Sight in 3.5? If he's devoted to the ranger, he may not even want to bother with the Shadow Dancer. Just a thought.
As for suggestions - not saying constructs and undead, but how about elementals, most things with blindsense and tremorsense, and scent skills. If this tiefling isn't flying (and fly hiding must be incredibly hard), an earth elemental knows exactly where he is. If the tiefling is alive and has been on the road, your average magic beasty lion with the scent feats from Savage Species knows exactly where he is within 60 ft. If he's facing a dragon, or something with concealment, he's boned.