"That's would take you a minute of conversation to earn a roll for, which you are fully allowed to do - just keep track of the rounds that pass during combat, and keep in mind that your ability to speak is being dedicated to the effort so you can't be casting spells with verbal components or doing your bardic performance in the mean while," for the player to realize that I was not kidding when I said "diplomacy doesn't do what you think it does."
The GM should exert more authority, and re-read the Diplomacy skill, which has several "loopholes" that reinforces the primary principle that the GM can do whatever he wants. For instance, the skill description says "Some requests automatically fail if the request goes against the creature’s values or its nature, subject to GM discretion." All the GM has to say is "that request is against this NPC's nature."
Players should never really argue with GMs. As a player, succinctly state your objection, let the GM engage you in debate if he wants to, but as soon as the GM rules you move on.