If he flanks them or if they're denied their Dex bonus for some reason, sure. Entangle provides Cover, not Concealment, and Cover doesn't prevent sneak attacks.
No sneak attacks just because they're entangled, sure. But Entangle doesn't prevent sneak attacks occurring normally, like a Fog Cloud would.
As an aside = there's a feat in the NBoF (for those who allow them ) called "Foul Sneaking", I think, that expands sneak attack to include nauseated, dazed, or entangled conditions.