Borderlands 2 features a "Mission Fast-Forward Function" which enables players to play together despite any variance in game progress.
This feature allows a player to join the game of someone who’s further in the story than they are and complete missions with them even if they haven’t reached that point in their own play-through. For example, if player A wants to join player B who has already reached a further area, the game places player A at the same point as player B, automatically bypassing the previous areas in order to allow the two players to play together.
Once player A returns to their own game, they start at the furthest contiguous mission progress point of their single player game. When player A reaches the missions they have already fully or partially completed through cooperative play, they will receive a prompt and have the option to either play through them again to gain extra experience and loot or to skip them completely and forge ahead.
This function will not give players any experience or loot for skipped sections they would have otherwise gained but the players can still make these gains through combat while completing mission objectives, as well as earn credit towards challenges.