We'll see what Rio has to say when confronted, but I wouldn't be surprised if she blames Kyouko. Seems like there's some important unsolved grievances between the three suspects, quite possibly towards Mahoro too. Maybe each of them saw Rei-kun with an unrelated fourth girl, and decided to take advantage of that to blame the one they have the most issues with. Maybe Rei-kun is cheating with all three of them. Or maybe they wanted to ruin Mahoro's marriage for some reason. The start of Ch.1 is fairly ambiguous, we don't know if Mahoro's accusation is specifically about cheating, or something else. In any case, I doubt there's a single innocent character in all this mess.
Also, Momoka could still be the owner of the cheater account, except the one she's doing it with is not Mr Hubby the Ass, but Rio's (yet to be introduced) husband.