I have two young children (boy/girl) who are extremely, extremely close and also fight like cats and dogs. I think mine are probably worse than some siblings because they are so close and also for whatever reason they really fight for my attention. We have to use a timer to determine how long each gets to cuddle with me, because God forbid one feels slighted.

The number one thing I will say that what you think you will do before you become a parent and what you do after becoming a parent, many times ends up being two different things. In the heat of the moment or when you are exhausted, you may allow things you never thought you would have, etc. I let my kids fight to a point because I'm really trying to get them to use their words and resolve their own issues. However, when it gets to the point of being physical or over the top mean or screaming, then I step in. My H steps in sooner because he has less patience for listening to them bicker.

You and your friend are going to have different theories and different ways of parenting. For the most part, there isn't a right or wrong way (unless true safety is involved) but it's really not worth arguing over, especially if you aren't going to change their mind. She will most likely figure out pretty quickly that letting the kids physically fight and work it out between them is going to create negative consequences. At least one will end up "hurt" and crying and running to mommy. Also, it teaches them that they can do that with other kids and it gets them into trouble pretty quickly in social and school situations.