My bestfriend and her son...?

My best friend is not only my "BFF" but also my oldest and dearest friend I have, I've literally been friends with her since we were 1. She's literally a sister to me. She has a 3 year old son and the father is no longer in the picture. My problem is that her son, though I love him to death, is SO rotten! She still lives with her parents and all three of them let this kid walk all over them. He's greedy and doesn't listen and is just as ornery as they come- all of this is due to the way they raise him. Recently I went to the mall with her, her son, her oldest sister and her mother. When we sat down in a restaurant to eat, she wouldn't put him in a high chair so he was just on free roam. He would crawl under the table and pull up on the side that her mother and I were on and take food off my plate try to drink out of my cup nearly knock me off the bench. This just went on and on and she wouldn't do anything but threaten to put him in his high chair. I was to the point that I almost walked out of the restaurant because I was so frustrated with his behavior. This is the kind of behavior that goes on all the time. They have taught him to eat his food by saying "I'm gonna eat your food..." and he will scream "NO!!!!" and snatch it up and shove the food in his mouth- that's just pure greediness!

I am aware that it is not my place to make comments on the way she raises her son but it is to the point now that I DO NOT want to go anywhere with the two of them any more. Like I said, I love her so much and I love her son just as much but his behavior is out of control and I don't know what to do about it. She always expects me to go places with them and I know that if I tell her why I don't want to anymore it will ruin my friendship. Does anyone have advice on this subject??