Well gosh. Isn't hitting assault? WHOA! NOT NORMAL!!! The more civilized we are the less we should NEED to resort to whacking one another. Those of you who said kids learn by example are right. If we hit our kids or each other, kids are learning that a good whack is a way to deal with dissatisfaction. That mother is going to be creamed by her kid, eventually if she keeps hitting him to make a point.
You need to talk to your friend and tell her you read something somewhere, or saw it on TV: smacking children is not only dangerous for the parent's relationship with the child, it is simply not acceptable as a way of disciplining children any more. Look up "Dr. Phil." Until you friend deals with the problem of her child hitting yours, you should not let the children be together. You really need to be firm about that.
I think I DID hit my kids when I was a young mother, not often, and not hard and always on their bottoms, but MY parents used physical punishment as a way to "get our attention" when I and my siblings were young, so MY hitting was learned behavior. I got "enlightened" by reading and living among people who DID NOT hit their children, ever. Your friend CAN learn better behavior.
Talking works. Depriving children of favorite things, like toys or entertainments works. Witholding LOVE is WRONG and barbaric. My children were raised to be good citizens, to have compassion for others and to eschew physical violence. Their more agressive tendencies were channelled into sports like fencing and soccer. They are both acclaimed good citizens, happy individuals.
Didn't mean to write a book.