Well, I didn't read all the response... so forgive me if I repeat something that has been said.
Navel piercings are discreet and easy to care for. Like wearing sexy underwear on a first date when you know you're the only one that's going to see it, it's an extra little confidence booster to make her feel attractive and hip without drawing unnecessary attention.
Well, she is 16. Call me an old fuddy-duddy at 35, but a 16 year old girl doesn't need to secretly feel sexy... and that is what sexy underwear and something like this do.
She can get it herself in a year... so she can wait a year. And in a year she can take herself to get it done and pay for it.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"