Amanda says “I'll give you game one if you give me game two. That way we won't go to time, and we can play post board.”

Is this Bribery?

Is an offer of a future result an incentive, or is this in the same category as “show me a counterspell and I'll concede… Thanks for showing me but I was bluffing.”

Discussion is welcomed.

I would call it Bribery. I see it as Amanda's friend saw Fred's deck and gives Amanda a scouting report. Amanda knows game 1 will take forever and she will probably lose. She feels her sideboard gives her a huge advantage and she can win the game but not necessarily quickly. So, she is trying to gain an advantage. Additionally, depending on when the offer takes place, she is trying to influence her opponent's record. She might feel she will sweep Fred, but needs him to win a game to improve her tie-breakers. Or she feels she will get swept and is trying to get a game win.

Overall, the situation feels suspicious.

Bribery - A player offers an incentive to entice an opponent into conceding, drawing, or changing the results of a match, or accepts such an offer. Refer to section 5.2 of the Magic Tournament Rules for a more detailed description of what constitutes bribery.

I fail to see the incentive being offered here. It's certainly more advantageous for one of the players, but it's not illegal to ask the question in my opinion.

However it's worth noting that the players could draw game 1 to play post sideboard. This could, in theory, be appealing to players in a known control mirror for the reason specified.