It's perfectly reasonable to leave a job after six months. Hell, it's reasonable to leave after the first day if you can see that life is going to suck.
But in real life, the next person to interview you will want to know why you left so soon. If your answer is along the lines of "I was being rotated through different departments for training and I didn't like the first rotation" that person will probably not hire you, expecting that you'll leave the first time things don't go your way.
You are correct that an applicant can make the wrong decision, but that's not what matters. What matters is how he responds to making that decision.
If the company is rotating him to positions that aren't his line of work, or what he studied for then I think it's a perfectly reasonable reason - he should be prepared to explain it more than "I didn't like it" - because that would be an issue, but otherwise I don't see the harm. In the end he needs to be sure that he is making the right choice in leaving and deal with the consequences if it's the wrong decision - or possibly be happier else where if it's the right decision.
As I mentioned previously, the 2nd cycle is more along the lines of what I studied for. More than that, I think I realize that I'm not meant for a BigCo and startups want a certain level of experience, and rightly so.
But in real life, the next person to interview you will want to know why you left so soon. If your answer is along the lines of "I was being rotated through different departments for training and I didn't like the first rotation" that person will probably not hire you, expecting that you'll leave the first time things don't go your way.
You are correct that an applicant can make the wrong decision, but that's not what matters. What matters is how he responds to making that decision.