What makes you think he won't step in or see it from his window? If what is being speculated is to be true about Dylan seeing Sopha/Sam from the window from the house, providing that you have enough Cuckold/NTR points and Sophia/Sam part is active, will L&P give you the choice of either have Dylan stay silent from and watch from a distance, or go down an interrupt them? That has been happening throughout the Cuckold/NTR choices with other scenes. In a realistic setting, people have walked in on or seen things from afar, putting them in an awarded moment. I'm sure it has happened to you a couple of times throughout your life. Also, Liam doesn't receive Cuckold/NTR points because it's not written into the novel. Now maybe down the road, L&P may change this thought.
L&P has said that he wrote Dylan's Cuckold/NTR because he wanted Dylan to have a "watching" fetish. And even though L&P removed "The Twist" from the novel, I'm sure certain elements are still in the novel. One example is Dylan wanted Sophia to feel sexy again. I think this is one of the reasons he wants to watch, so he can see it. To me, that should have gone to Liam on the Filthy Side.