The first time Mrs. Saeki came clean about her husband that was imprisoned for criminal charges, Mr. Hokama asked for permission to look into that man. The lady granted it. She also was curious about her husband's date.
Because neither Mrs. Saeki nor Heisuke visited Mr. Saeki for more than a decade, they didn't know anything about his well-being.
The prison was certainly a harsh place to be in. The mother and son didn't care at all about the man's fate.
Mr. Hokama just needed to rely on his connections. The news about Saeki Tetsuo, Heisuke's father reached him back in less than a week.
While Mr. Hokama had already conveyed the news to Mrs. Saeki, Heisuke had yet to hear about it.
Still in the discussion, Mr. Hokama briefly glanced at Mrs. Saeki before he opened his mouth again.
"Heisuke, I understood that you only want your mother to be happy. I am serious about her. I'm not a coward who lay a hand on a woman or a child."
So, in the later chapters, I'd still refer to Heisuke's mother as Mrs. Saeki since we're already used to the address.