The diplomacy rules, as written, are broken, although you could have ruined the plot if the victim had failed vs a Charm Person spell as well.
The difference is, they get a Save v Charm Person. Diplomacy rules for changing the attitude of someone from
Unfriendly to
Helpful are an unopposed skill check. And neither the level nor ability scores of the target come into play at all.
Incidentally,the way I figure, an 8th level bard has 11 ranks in Diplomacy, +2 from 5 ranks in Bluff, +2 from 5 ranks in K. Nobility and Royalty, +2 from 5 ranks in Sense Motive, +2 from Half Elf, +3 from Skill Focus, +2 from Persuasive, and +4 from Cha to give a total modifier of +28, which makes a roll of 45 occur on a 7. Lower if you use the Improvisation spell.
I didn't Skill Focus, but the character has a +2 charisma bump from Half Satyr (20 Cha at this point), the +2 RA from his mother's side (He's Half Elf/Half Satyr ), and a +2 Charisma item, taking his effective Charisma mod to +6.
So he's +25, total, on the check. Even if he were the +28 you describe though, he'd still need a 17 to reach 45. not a 7. My 45 result was a natural 20, and the DM rolled a resist check of some kind and popped a natural 1.
Defying all odds though, it's not the first time we've matched dice that way on a Diplomacy check. It's not even the second time. And it always seems to come up at a key moment.
In this case it was a prisoner we'd taken from a group that had kidnapped a small child. My character had been in Avariel form for the battle, so he didn't realize that the bearded, horned hairy thing talking to him was the same guy who'd whupped him unconscious a day earlier. His group leader had absconded with the kid (DM fiat: He gets away because he has to for the plot), and his other companions were killed in the fight.
My character isn't an intimidating type, so I tried Diplomacy. Loosened his bonds so he could eat, played for a little Stockholm syndrome, if you will. I just wanted him to tell/show me where the kidnappers were taking the kid. I played on the fact that his group leader had abandoned him on the field, and was now taking credit for the success that everyone else had died for. I laid it on thick. Then I arranged his "escape", and went with him. We got there, with my companions following a mile or three behind. Ranger, speed tracking from out of sight, and me leaving an obvious trail. A few days on the road.
We got there, my associates showed up, the prisoner pulled a Houdini and vanished into the shadows (DM fiat, no chance to spot, track or follow). We were planning our assault on the evil temple when our "friend" shows up again, kid in hand. We make our escape before the alarm goes up, and the pursuit doesn't catch us. DM fiat again.
Have I mentioned how much I hate DM fiat? This DM doesn't do it much, but when he gets on a roll, well, at least he doesn't use it to be a killer DM.