Could you paraphrase the underlined part in the following:

Britt Blackerby, his attorney, had tried one last time to talk him out of it. They’d been friends for years; six months ago, when Paul first brought up all that he wanted to do, Britt thought Paul was kidding and laughed aloud, saying, “That’ll be the day.” Only when he’d looked across the table at the face of his friend had he realized Paul was serious.