Your friend found a point where he was satisfied with what he got. Maybe his trading experience so far, about how many times he missed the opportunity to take profit made him think that if he could close the trade when he felt satisfied, then he could get consistent profit. So he realized that if he could control his greed then it could give him a good profit.

And I feel that the average trader experiences the same thing when it comes to controlling their greed. they learn from their experience that they can actually get a decent profit if they can control their greed and not trade beyond what they can afford.