Well...That depends on whether you're asking for your friend or for you.
While it's a totally normal "phase"...if it's not nipped it becomes a an actual pattern of behavior. Nipping, incedentally, can take days, weeks, or MONTHS of consistent teaching. How long it takes depends more on the child and their own personality and current struggles and challenges, then on either parent or method. UNLESS there are inconsistent reactions.
Two of our very good friends we barely get to see anymore...because they think their son hitting is "cute"...they encourage it. (By laughing, turning it into a wrestiling match, or using it as an emotional barometer "oh are you bored with that, honey?" "oh, you've had a hard day, haven't you? Let's do "x" fun thing", or by blaming other kids who irritated him. Maybe 1 out of 5 or 10 episodes of hitting has negative consequences.
Sigh.
WE though, are not his parents, so even though half the time his hits other kids because he wants his parents attention for himself (because at this point, none of us are willing to deal with it.) Johnny hits our kids, we leave. Which is more positive reinforcement. But quite frankly, I'm not using my kid as a punching bag.
In our family,
you hit: you don't get what you want
you throw a fit: you don't get what you want
you whine: you don't get what you want
;) And ALL of these get timeouts. Scooped up and on timeout. For our son, and for us, this has worked. All families are different.
Dealing with tantrums and hitting is never fun, but at least when it's your own child, you have the right to teach them. My sincere hope, is that your friends, are willing to teach their own child. Otherwise it can make continuing to stay friends very difficult.