All the rules say is this:
Rulebook v3 p19 wrote:
No character may gain a reward from a given scenario, adventure, or Adventure Path more than once.
There is no prohibition against repeating a scenario you've already successfully completed, you just can't get the reward for it. But the boons you acquire while playing you can keep.
Depending on how far ahead you are, going back will probably make the game easier. Mike would probably tell you this is the "social contract" part of the game. They aren't going to tell you that you can't do it. Just realize it might not be fun.
If you are talking about going back more more than 4 or 5 scenarios, I'd suggest instead trying out a new character with your friend while they catch up. Then scrap your new character when your friend joins your party.