Well, if one of my kids had a friend that had grown particularly attached to me, they could probably call me “Mom” or “Mama”. Likewise, if I was in a store, and someone was addressing a child of mine, in reference to me, they could properly refer to me as “Mother” or “Mom”; for instance, the grocery store I shop in frequently has free cookies. If my six year old wants one, a lady behind the counter will sometimes look at me and say “Is that alright, Mom?”. That’s fine. She’s making it clear to my little girl that she’s asking my permission before allowing the child to take a cookie. I can even kind of see it in a friendly, somewhat familiar term if I’m pregnant (I’ve heard comments like “You look tired, Mommy”). But for random people to just start calling my “Mom” or “Mother”? Uh, no.
Perhaps the book was an old Emily Post? Some of the older books can be unintentionally funny, and very anachronistic.
StG