I still don't know if I should have done this but I had had it!
Background info:
I have 5 children, they are not perfect. My friend has 1, he is.

We are very good friends, and I love her dearly. But NOT her son (9 years old.) Everytime they come over I catch him hurting my children. He is bigger than all of them, and he loves to bully them. I've caught him spitting beans in my young daughter's eyes (she's 4 years old), repeatedly kicking my small son in the groin (6 years old), and the whole time he is here he usually has my 8 year old son in a headlock. I normally ask him politely to stop hurting them if I see him doing it, and let my friend take over from there. But usually he just lies to her and she will only threaten him, or some benign punishment. She hasn't mastered the art of seeing through children's "stories." Now my children are no saints! I have many stories of them, and I am not blind to their behavior. But this is a story about him. I will call him VB: Violent Bully.
Okay, now here is "the incident." One day last week my friend (this boy's mother) showed up out of the blue with her friend and this friend's child in tow. I was embarrassed because the house was a mess and so suggested we sit on the front porch furniture. The children took off for the back yard. I am making small talk with the women when I hear a scream. I know exactly which child of mine it is and that it's a "real-pain" scream. (when you have THIS many kids, you have to know how to read the screams!

) I get up and excused myself and ran to the back. I find my DS1 scuffling with VB, and my DS2 crying in pain and holding his head. I asked what had happened and VB (for once) didn't say anything. My DS1 told me that VB had punched my DS2 in the stomach and then pulled his hair. And DS1 was trying to defend my DS2 from any more attacks. I could tell from the look on VB's face that he had done this, and he didn't try to deny it this time, so I told him this: "I am sick of you coming to my house and beating up my children. Every single time you've come you have hurt one of them. You are bigger than them, and have NO business purposely hurting smaller children! I do not want you to come and visit my kids again if you cannot behave yourself. I am not happy with you, and I expect this behavior to end, NOW! You are going to sit in this chair right here, and I will be telling your mother exactly what happened. Do you understand?" The whole time I was talking to him, I was shaking. I was so upset with him, and just tired of having to protect my own children in their home. I left him in the backyard, and hauled the other children up front to keep a better eye on them. We got to the front and my friend asked where her son was. I told her that I had caught him fighting and asked him to sit on a chair for awhile. She answered "Well, good!" Then I served them some refreshments and called to the VB to come to the front yard. He came and sat by his mother and wouldn't play with the other kids. Which was fine with me. Then the visit (thankfully) ended, and they went home.
Since then, my friend has been strained with me. But what would you have done?? Maybe I should have called her back (having her walk through my disastrous house) and let her deal with it? Or just told him to go sit with his mom?? I didn't know, I just couldn't take watching my children get beat up in their own home for no reason--yet again! And if you're gonna mess with the cubs, don't you expect momma bear to get a little riled up?? What should I have done??
(PS: Something good did come out of this, my DS1 finally stood up for his younger siblings, and DS2 is not bugging him this week.)