A: It’s great that you have already raised this issue with your boyfriend's mother. I think this was the right thing to do. If your boyfriend's mother was a completely wise and sensitive person, she probably would not have hit your son (even lightly!) to start with and she would not have hurt feelings when you asked her not to hit him. However, she does not see the world in the same way that you do. Your boyfriend's mother may have been hit by her own mother and grandmother and thinks that it’s natural. By hitting your son, she is telling him and you that she feels like you are both part of the family. The basic idea is that she feels close to you, but you still need to prevent your son from being hit. This is your challenge here.
I suggest that you do something to show your boyfriend's mother that you appreciate her hospitality and her intention to discipline your son as if he were part of the family. Buy her flowers or bake her cookies or make some similar gesture to show you wish to make peace. Explain that you don't want to hurt her feelings and that you understand she was trying to "help" raise your son, but that you have very strong opinions about raising him without hitting. Say that you welcome her coming to you and complaining about things he does, such as fiddling with her cell phone. Reassure her that you value her input, and that you'll take it from there.