It's not easy to explain Bona, but she's hardly unique--Brocka noted how professionals and even upper class women hang around Nora; Isaac Bashevis Singer wrote about a mistress, a beautiful, intelligent and courageous woman who unaccountably cooked for her man's many lovers in The Manuscript; Henry James did a whole novel on such a character (Washington Square); so did Francois Truffaut (The Story of Adele H--the daughter of Victor Hugo, no less).
I figure Brocka saw it was not impossible, and ran with that.