Obviously he doesn't care, he says as much when Silvia first points out the coincidences with MC and Nicole, but he dismisses the idea of anyone being interested in her if he's not. It's the second time she points it out, during the business trip, and MC's reaction in the office, that he begins to consider the possibility, and starts the tests.
What I don't get is what is John's end goal. He seems to be grooming MC to become "heir to his empire," which would kind of make sense if he saw him as a future son-in-law, but he definitely seems to believe Chloe and MC will never marry, regardless of player's choices, so why keep him on? Furthermore, he suspects the affair, trying to find ways to expose the smoking gun, rather than just firing MC, forbidding or sabotaging the MC-Chloe relationship, demanding the house key back, and thus removing any excuses for MC to ever go to the house and be with Nicole. Obviously him offering Silvia up to the MC was a test, but I wonder what he would have done if we players had the option to accept (stealing two women from John would have been glorious, missed opportunity). So clearly he's not trying to stop the MC-Nicole relationship. My only guess would be he's a puppeteer, so as long as it doesn't lose him money, he lives for the joy of manipulating people, and watching people squirm, thus the affair offers him two new toys to play with: MC and Nicole. His lack of interest in her, therefore, probably is less to do with her looks, but actually due to her having lost her will years ago due to his manipulation, whereas Silvia, despite being completely submissive towards him, holds onto her will through her dream of Malcolm, thus his "searching" to keep her motivated. Thus an affair has rekindled a fire in Nicole's spirit, which is the plaything that John sadistically craves.
