Split party dilemma

Me (DM) and my players (5 total) have had a game going for 2 and a half year from level 3 all the way up to level 17 and now we are at the climax of the campaign.
1 of the PC's has always been an agent of the BBEG which is an all powerful eldritch entity who is trying to consume all life across all universes. Currently the party is on it's final quest to stop it. Now the PC who is an agent of the BBEG is made a preposition to the party. Join him and become all powerful and get your own little mini-verse where you are the ruling god. Or stay and perish. 2 PC's joined him and 2 PC's didn't and now the party is split. I personaly did not really see this coming as DM as I thought he was going to join the enemy at the final fight as a big reveal but he choose a different approach.

The issue is now that I am unsure how to continue. I personally don't wanna make it PVP, as I believe that will just create friction at the table between players. I am looking into alternative ways like going with the majority of the group and allowing new characters to join for the other PC's. But even that will leave a bit of a sour taste. Do you guys have any idea's on how to handle such a situation? Also I am already talking with my players on what they want and if they would find a solution satisfactory, so I don't just drop this on them.