She lets him do it. Usually.
A child has to be permitted to make these choices, which, in the long run, don't harm them. They help the child figure out who he is.
Of course, as with everything, there are exceptions... If I know that a style choice will lead to teasing at school, the least I will do is warn my child that it could happen and then let them make the decision.
I also make them wait a few days if the change is dramatic. I would not want them to make a rash decision and regret it soon after the change was made.
As far as fashion choices go, I usually stay out of it, other than saying, maybe, "just so you know, black shirt and blue shorts do not really go well together, you might want to think about changing." If we are going out and I want the kids to look nice, I have been known to put my foot down. Usually, on those days, I pull the kids' clothes out the night before and lay them out so the boys just put them on without thinking. Usually, this method works, but not always.
I am writing this because I have recently been there and done that, again. My 6 year old, who has had long hair for most of his life, has been saying he wants to get a shorter spiky hair style. I put him off for a few weeks, but a few days ago, he became very sure of his desire for the new look. So, I made the appointment, we printed out a style picture from the internet (love google images) and yesterday he got his hair cut.
It was not easy for me. I love his hair long, and it is kind of his signature look. But I knew I had to let him do it. In the end, it looks great. Totally different, but fabulous nonetheless.
He feels proud that he made the change and hopefully he'll take this experience with him as he grows and matures.