So he's a dedicated face-man who's very good at being a dedicated face-man? Rockin' for him. We'd need to see the build to be sure, but it looks like he's sacrificed everything else to be a great negotiator. As long as the game is balanced so that some encounters are primarily social and some are combat and some are skill-based (with skills he an't got), this is just peachy. He's no weirder than the guy with the high strength and weapon focus in the sword he uses all the time. He's trying to be good at something.
DC-wise, here's the rule of thumb I use:
Generally, 14+Level is a good DC. Now, sometimes it's supposed to be easy, and sometimes it's supposed to be hard, and those mean that you modify, sure. This doesn't mean "This average lock would have been DC15 at first level, but now it's DC20." It means that you, as the DM, design adventures so that they're running into higher-level stuff. They're not worried about breaking into the homes of people with average locks anymore. They're breaking into the homes of people with stronger security systems (DC20).
Now, that 14+Level rule is there for not-critical-to-whole-plot checks where it is expected that the characters will not have the chance to take 20. For example, Spot checks. Sense Motive checks. Casual Diplomacy checks. Jump and Climb checks. Things where the team can't do it over and over again, but it's not like the whole plot is destroyed if they blow one check. If it's critical, you lower the DC a bit, because you want them to have a better shot of, uh, not utterly failing in their quest because they blew one roll.
If it's something that they can take 20 on trivially, the DC should go up -- you'll note that most locks and traps have DCs that are high, because it's assumed that the party will be taking 20 to disable them.
So, for a 6th-level party, a nice skill DC would be 20. This means that:
Bluff +17 -- 90% chance of success.
Diplomacy +23 -- 100% chance of success.
Intimidate +18 -- 95% chance of success
Gather Information +17 -- 90% chance of success
Sense Motive at +14 -- 70% chance of success
So, for an average encounter, this guy is majorly tricked out. That DC20 is really an approximation for some of these rolls, because some of them will be opposed rolls -- the Sense Motive, Bluff, and Intimidate checks all involve opposed rolls, so there's more randomness. DC20 assumes that an opposing character with a +10 from abilities and skill ranks just took 10.
So here's what I'd say:
1) If this is a legal build, don't penalize the character
unless this is not the kind of game you want to run. "I want to run high-combat, with conversatoins as just flavor-text to get you to the next combat" is perfectly legitimate, but this is an awful character for that kind of campaign, because he has the ability to affect the plot with his social skills.
2) This character can be a blessing, because he's powerful enough that you can really unleash a nasty skill-based opponent. Ordinarily, an opponent with a +20 Bluff check can't really act, because he could say "I'm actually your father under the effect of a Polymorph spell -- I only look like a black dragon" and the party would fall for it, and then have to roleplay "The black dragon sounding believable" when it says such idiotic stuff as that and succeeds in its bluff check.
3) Allow this character to succeed in trivial cases. Let him bluff his way past the guard easily, when the other people have to sneak by or beat up the guard. Let him find out stuff that might not have shown up until next chapter. Plan for these kinds of skills, both in terms of "Special Challenges designed to let this guy shine" and "What do I do if he succeeds here?" I once had a character turn an opponent from Hostile to Friendly with a diplomacy check of 37. Had to do some fast thinking, there. Changed quite a bit of that adventure.
4) Read up on these skills. Know the rules. Diplomacy has limitations. It can turn someone Friendly, but that doesn't mean that they're charmed. It means that they will deal with you in this situation as they'd deal with someone they liked. If you've just been caught robbing the king's vault, and Diplomacy turns a guard Friendly, that means that he's going to give you a chance to come peacefully, just like he'd do if he found a friend robbing the king's vault. He doesn't say, "Oh, okay then, off you go, good luck selling that loot!" By the same token, Gather Information only works if that information is known by someone in the area, and it takes several hours.
5) Consider modifying the rules to let the player have more choices. I use a "-5 per time increment shorter" approach for some of my players and things like "Research" or "Gather Information". He can take the usual "several hours" as normal, or he can drop to "tens of minutes" for -5, "single minutes" for -10, or "single rounds" for -15, or even "single round" for -20. At a +17, that guy would find out a ton of useful stuff in two to five hours, but he could also find out
some stuff in just one hour (the tens of minutes setting, -5 penalty), giving him the option of hurrying stuff up. If you put a time limit on things ("We've got to find out where the assassin's guild is located by midnight, and it's already 10:30!"), this means that the checks get harder, and the PC gets to shine by doing something that would ordinarily be just about impossible.
This build, if legal, is perfectly viable. It's just something you have to plan for, just as if the character had, well, spells.