Dear Distressed Dad,
I would write a list of your fears and address each separately to see what she can do to alleviate the fear in you or prove she has made a considered choice. For example:
You might fear infection. Have her read up on this and pick her piercing service with this in mind. Have her discuss the post piercing care with you. Let her do the research herself.
You might fear she regrets the hole. See if she might be willing to wait a few months to see if this idea is still appealing and not just a whim.
You might fear that this draws sexual attention to her. She might show you that today these belly piercings are as common as ear piercings and that it doesn’t signal that you are a hussy or easy, the way a parent might imagine.
Remember, it’s your child’s inside character that matters. So if they decide to go goth and wear black lipstick, or jump on the piercing and tattooing look, we have to show them we love them regardless. They are finding themselves and their identity. Love them while you offer some guidance. But in the end, you cannot control them.