Might be a bit too late for this idea, but:
If there were 2 different MCs, one with the brunette wife cheating, one with the MC getting his friend's blonde wife.
(meaning there would be 4 men and 2 women in the island).
Then after some time exploring both routes, the 6 people will find themselves together and continue the story.
OR (more interesting idea):
He could have made both routes together right from the start, but locked after player choice, which would have made the game more interesting instead of 2 entirely separate stories together.
How would he do that?
1. MC can choose to bully his friend or not at the start of the game, which triggers or eliminates the Bully route.
2. MC can be nice with his wife or not at the boat, which triggers or eliminates the NTR route.
If both routes are triggered, the MC will bully his friend by fucking the blonde wife while the brunette wife is fucked by the 3rd dude.
If none of the routes are triggered, either the MC dies, or the game ends, or we have a boring couple story with your friend being bullied by the white-haired dude..