You are trying to blame a Mother for doing what she is supposed to do. What did you expect her to do? Did you expect her to say, "Oh, I am sorry sorry to interrupt you...please continue your play." You did what you were supposed to do...get the hell out of her house. The next thing she should have done is call your Mother. As far as her son is concerned he should have been punished for disrespecting his family for screwing around with his gf just steps away from his family.
Is it wrong? Probably not. Is it stupid? Absolutely