It's based on the party leader at the time. Whatever advancement the party leader has made, that's the furthest point that the party will go to upon entering Coil. Therefore, if any single member of the party has reached a later turn during the week than the party leader is at, the instance won't begin. So, he can join someone else's group to go back into Turn 2 and it'll be perfectly fine, but if he beats Turn 2 with that new group, he cannot be a party member with his first group to do Turn 2 with them. The only way, until reset, that he'd be able to go back into Coil with his original group would be if he became party leader and started Turn 3, thus forcing the party to skip past Turn 2.