I am horrified that another adult didn't step in and put some freaking lotion on her. Poor kiddo. And every burn can lead to future problems. I would be so upset. On trips like that, I make sure any kiddo without an adult with them are ok (sunscreen, restroom, help with a project, etc). I was fried sooo many times as a child, I am sure you are like me, and don't want that to happen to your kids!
Have you seen that spray sunscreen, the stuff you don't have to rub in, just spray and it soaks in? Its pricier, but my 7 yr old can do her front herself and then I encourage her to ask an adult she knows/trusts to supervise her spraying, then to help with her back.
Your email is perfect. You were shocked when you saw your D9, so you can emphathize with how he felt when he saw her as well. I think you did a a great job agreeing with him and neutralizing his anger (which is really a cover for his worry about D9).
