What does bill do when you beat blaine in FireRed?

In FireRed, Bill doesn't do anything special when you beat Blaine. He doesn't change his dialogue or his team composition. You can still challenge him for rematches after beating the Elite Four, just like any other Gym Leader.

Bill's main role in the game is to help you with the PC system, and he doesn't have any specific interactions tied to beating Blaine.