Should I mind my own business? And how?

Ok. So I am a 20 something year old woman. I work in the same place as my mother and we share an office. There is a man who comes in sometimes who occasionally acts as our superior. He and my mother have known each other for years. They are both married but alway flirt with each other. This man has a tendency to cross the line and I sometimes end up telling him to back off but it s usually all fun and games. Today, however I felt like they went way too far. My mother took something of his and put it in her shirt. He got my attention, like he usually does before doing something silly, and then went to reach into her shirt to get it. It instantly pissed me off. I ended up telling him to keep his hands to himself in an aggressive tone while he argued that I should mind my own business. My feeling is that my own father doesn t touch my mother that way in front of me. We are at work. And I have enough respect for my mother not to do things like that in front of her and expect the same in return. I really felt like telling him completely off but managed to stop myself. I m not sure how to bring this up to them or if I even should. Help.