You don't have to put your hands on a child to discipline them. Discipline also need not be immediate. Lots of times you get more mileage by making them sweat it for a while.

He didn't slap her. The author made that up, at best, it's wildly conflated. He put his hand over her mouth.

Lastly, recognizing that something is not.al behavior is not the same as endorsing said behavior. I don't know what happened with your friend's kids, but you can't assume that everytime someone says, "this is normal behavior" it means the kid is allowed to continue the behavior indefinitely.

Sometimes you have to let a minor unwanted behavior slide for a while in order to prioritize a more problematic one. Lots of kids don't do well, especially when that young with being constantly punished for every little thing. It's overwhelming and does crush their self esteem. They learn they're not good enough. With kids like that, you tackle one thing at a time. Two at most.