In my state in the US, unbelted people can be fined. If it is an adult, then that individual is fined; if it is a child, then the driver is fined.
Regardless of who is legally responsible, I feel you had the right to speak up. Just because the other person is willing to risk having their brains bashed in, it is no excuse to endanger others.
Same as most things in this world. I believe you should be free to do as you like as long as it does not harm, or potentially harm others.
The one being unreasonable is your friend. Especially after you explained your valid concern.