Ideally, for starters, the previously angelic and carefree mother would feel profound shame for having been fucked by a young man her son's age.
At some point some arrangement, whether voluntary, coercion or seduction would be reached that it would be their secret. Also the mother would have a misguided sense of relief that at least she did it for her son and things would improve after that. Naturally Mr. B would completely betray every single one of their agreements, immediately bragging to his friends, treating her son even worse and demanding more encounters, each time promising to be the last.
Completely humiliating her, on top of the son being powerless to stop it AND possibly even have Mr. B force the son to "play along" with the story that things are getting better for him as a result of his mother's sacrifice, a sacrifice she slowly starts to take pride in as a "good mother" which itself would backfire spectacularly down the line. Basically mind-break after mind-break.
