eastendgirl, boy, do I know about string personalities and was nodding my head about your friend's comment about it serving her ds well in adulthood ... but her ds needs to be made aware at every turn that aggression (whether towards her or other children) is a big no-no and will not be tolerated - in this culture anyway.
My dd is headstrong and I definitely believe that going head-to-head with her is the wrong strategy. As a result, I am forced to always consider my/her requests and yes, she probably gets away with a lot more than other children because I have to pick my battles.
But aggression is one battle I WILL pick and will fight over and over again (lol!). Thankfully dd knows that she crosses the line on that one.
In letting her ds know that he is wrong to hit her, your friend does not have to clampdown really hard on him. I don't do time out or punishments. I just hold dd's hand and tell her not to hit and to be gentle, even kiss her hands sometimes. If she persists, like you, I put her down and walk away. But she always get the message that it is wrong, no matter how many times she does it.
It does seem like your friend is worn down (I understand how wearying dealing with a strong personality is) and probably condones it because of the macho culture. How asking her whether it would be different if her ds were a girl? The answer might be telling.
Of course, it is up to her to set the boundaries for her ds. I frankly don't know if her ds will "grow out of it" if he is not given consistent guidance. He (and your friend) may end up learning the hard way, if other people (like you) start distancing themselves and their children.
But definitely, her ds' home environment needs to be sorted out as a priority. It is not reasonable to expect children to behave when they are subject to stressful situations.