Nathan eventually found out about Bella's divorce suit against Carter, when Carter came to his office that day. When Julie tells him that Carter is coming, Nathan already has a feeling that this time there must be something big going on. He found out from his investigator, that there was something going on in Saco, involving Bella.
"You don't have to keep me guessing," Nathan grumbled to his investigator. "Is Bella going back to her father's house?"
"Could it be?" His investigator asked back.
"Of course not." Nathan retorted.
That's why when Carter entered his room stumblingly, he knew immediately that Bella had no intention of returning to New York City. "How are you, Carter?" He asked casually, as if he hadn't sniffed out any news. "I heard from your mom that you were so busy that you forgot your father's birthday. She was dissapointed."