Is it ever OK to discipline your friend's kids?

Last week I spent the day with a dear friend, a woman who I've known and been close to since I was 18 years-old. Our friendship grew deeper in high school and college, we saw each other almost every day for nearly a decade. As adults, we've been bridesmaids in each other's weddings and attended our children's christening(s) and bris milah together. I can call her at any time, with any problem. I can her pretty much everything, and I'm quite certain I'll know her until I die.

But I can't discipline her child.

I figured this out on last week. We were out and she warned me that her little 3 year old was wound up. She also warned me that she was distracted because she was on-call at work. At one point, her son was running away and around and she was texting up a storm. For no reason I could discern, the little man came over from his whirlwind tornado and hurt my little one who was playing peacefully on the floor.

"wwaaahhh" went my little one and off the little Tasmania devil went again, running away at full speed, in circles. I scooped the little tyke up and set him down and looked my him dead in the eye and said firmly: "You need to stop. You need to stop right now."

He did. But I saw the look on my friend's face and it told me I had crossed a line. I didn't bring it up and neither did she, but the rest of the afternoon was little weird and I made sure not to interfere again.

Though I'm still not entirely positive that I did anything wrong (I mean the kid was hurting my child and acting up, and I simply told him to stop), I suspect there's some etiquette I don't yet understand. There must be a rule that says you're not supposed to reprimand other people's kids, even if those kids are being obnoxious. But believe me I have a friend who has no problem at all telling my kids when they have crossed the line with her (each person has their own personal boundaries and mine with my own children are very wide).

It's not the first time I've run into this problem. I have had plenty of incidents at parks and such where other children have just for not other reason than to be a brat, walk up to my child and pinch or bite them. Seriously! I do not (DO NOT) keep my mouth shut.

Here's the thing, I have no desire to tell other people how to parent, but when their children are behaving in an unacceptable way, what's an adult (bystander) to do?

Do you let the kid scream in your face? Do you allow a 4 year-old to wander into potential danger on your watch? I'm honestly perplexed by how to behave in these situations, but I'm sure everything I've done up until this point has not only been wrong, but somehow offensive too.