What do you do when you have a burglar doing that kind of sneak attack? Give him a burgling mission. If he can play it well, don't punish him. Sneak attacks are, by nature of the limited number of attacks and the Damage Save system, not nearly as game breaking as they can be in other games. I play an Assassin who can almost freely get 7d6 Sneak Attacks with a weapon that has "Armor Piercing: Yes" on it. Let the Burgler Burgle; he might find he's left out of the loops when things don't need to be murdered, but unless he's outright ruining you game (Which sneak attack is hard to do compared to many of the OTHER options you can pull off with a high number of combat feats) just treat him as a guided missile. He can probably only get off one sneak per turn in optimal situations, compared to people with Cleave and Blade Sweep and other things that allow them to attack multiple enemies and do damage nearly comparable to sneak attack anyway.
I'd say make sure the rest of your players have their own awesome thing about them, and just let the burgler burgle. 7d6 sneak at level 6 isn't all its cracked up to be unless you're only fighting one thing at a time. Throw some mooks, toss some standards into the mix, maybe add a pair of specials in a single combat, and it'll balance itself out.