After knowing about Ken cheating on him, he knew to himself that he needed to know the truth regardless of what Lucas told him was real or not he would still want a piece of evidence. It was not like he doesn't believe in Lucas but rather his heart was not ready to be broken by the one he loves the most. It would be a lie to say that he doesn't want to deny it, after all the years that they had been together a single word about Ken cheating on him won't be believable right away.
At the same time that he was trying to look for proofs, Dr. Lin suddenly sent him an email containing some pictures with it. When Ethan looked at the pictures, he then saw the proof that he was looking for. It was as if his world stop after seeing the pictures of Ken exiting CGH together with someone whom he doesn
