IMO, it would depend on what the sorceror was trying to do and how he was trying to do it. Intimidate if he was trying to get the bandits to let them by by convincing them it wasn't a good idea to fight them; bluff if they wanted them to believe that they weren't worth attacking; diplomacy if they wanted to try to make it worth the while of the bandits not to attack them.
The Bluff (depending on what the bluff is) would have a DC = 1d20 + Sense Motive + 5-10 (+20 if it's way out there), or probably about 23-30.
The Diplomat DC would be at about 25, to convince the bandits (who are probably hostile) to leave you alone.
The Intimidate check would come to probably 17 minimum, probably quite a bit higher because of circumstances (surrounded by friends, obviously better equipped, possibly higher level, etc).
In this case, the best plan would likely be to try the diplomacy skill instead of the Bluff (in some cases) or the Intimidate (in this case). I'd allow any of them to work, but these skills cover different approaches to doing the same thing (in this case, getting the bandits to leave you alone), perhaps even letting the player to combine them (bluff to convince the bandits that an archmage is waiting in the wagon train who would kill you with a single word if you disturbed him), but I wouldn't say straight-out that it's a skill check of x type.
Just my opinion, of course.
Magius out.