So, I have this big encounter set up. It's got everything; there's monsters and traps and a big set-piece battle to bust into an enemy fortress! The players are all ready to go, and then the warlock says those dreaded words....
"I'm gonna use Diplomacy."
Oh god. He's level five, and he rolls a total of 32 (18 on the die + 4 charisma + 5 trained + 5 power bonus), so now I have to deal with it.
Well, he tried to talk the hobgoblins into letting him pass, and his check result certainly supported it, but I didn't want to let this big set piece go to waste (and there wasn't really any way to insert it later), so I had the hobs accept his claims of friendship but stand firm by their orders to let none pass, and therefore allow him to leave peacefully if he chose, but he'd have to fight his way in anyway if he wanted to go on. (Which they had to, so they wound up doing the fight anyway.)
How do I deal with this sort of thing? I felt bad that I essentially screwed him out of any benefit of his massive diplomacy roll, but at the same time Beguiling Tongue is an encounter power! So I can expect to be seeing this a lot in the future.
How should I handle trying to talk his way out of a fight?
I guess making one good talk roll should count sort of the same as making one strong attack roll, so it should remove a minion or two, or demoralize one normal monster, or something else that's approximately what one good attack roll would do. Even then I know he probably won't be happy with it, because I know he's angling to try to bypass the battle totally. Intimidate has some rules for how it might interact with combat, but Diplomacy and Bluff? I have no idea.
Honestly, I think that you handled it fine. It's absurd for someone to expect they can beat an entire counter with a single dice roll. Allowing them a chance to talk and allowing the players to leave freely is reasonable, maybe even generous.
Diplomacy is more of a skill to be used in social situations where hostility isn't already present, frequently part of a skill challenge.
If he wants to use Diplomacy, ask him what he is trying to get out of it, and what he actually says to them. Don't expect your player to be as glib as his character, so really look at the intention and the goals of it and how reasonable it is with that enemy.
Then consider the likelihood of them doing such a thing. That's going to be the basic DC there.
And figure in other modifiers, such as they are of a different culture and diplomacy might not follow the same rules (maybe -5), and they began as hostile (-10). Maybe that initial large success can help get rid of the hostility for the moment, and get them to listen (if there's a compelling reason... or bribe).
And as others have said, you can also allow it to open up a skill challenge. It totally isn't what was planned, and you might not feel comfortable with that route, but it's a valid route to pursue. You could make Diplomacy a *part* of it, but all the successes should *not* be just that skill. Other players need to contribute. Maybe Bluffs, *possibly* intimidate (or maybe that could backfire), or maybe History to spot and know something useful about this particular Tribe, or an Insight to get a feel for whether they are more loyal to their boss or some coins, or maybe one of either Nature, Heal, or Perception to notice that these orcs look very sleepy and hungry...
At least you didn't just totally ignore them the right to role (which some DM's would do).
Even worse, I had a DM have an NPC club me on the back of the head, automatically hitting, no save (3.5 Ed), and interrupt my attempt for talking our way out of a capture.