Hiya,
It's not a gender thing, it's a parent thing. You will react differently to your Son's behaviour to how your friend reacts to her daughters behaviour.
You probably don't realise it when you do it, but you must in some way re-inforce your childs good behaviour when you talk to him, via your tone of voice etc.
When your friends Daughter misbehaves, she can probably hear her Mum's voice getting strained and see her temper wearing thin - which is the reaction she wants.
Chances are if you were to have a girl, her personality will be closer to your sons than to your mate's daughter, just because you are the Mum!!