I don't understand what the blackmailer could have to destroy their life like it did in the bad end, I mean unless he had a camera in the room or they were stupid enough to do it without closing the curtain, then the only blackmail material they could have is that two of them are infertile and he visited his friend late in the night and even that is a stretch?
Yeah someone could start a rumor that there was an affair between two couple of friends, but that is just a rumor and nothing can be proven. I could start a rumor that the prime minister of japan is having an affair with the wife of his best friend, but that wouldn't have any consequence.
Also I doubt it could be Ayano or Jona, since destroying your own life because the mc didn't respond to the blackmail is extremely stupid. (Bad end)