Spot on!If you're concerned about being polite, you'll do fine. Edit heavily & proofread before you send. And don't do it in public or say anything to shame her, certainly not to start.
I often start by saying how lucky it is to live in a place that still has a native population. How taking a single turtle from the wild affects the future population and removes large numbers of generations. How they don't transition to captivity and often die within the first year. How it's possible that the same wild turtle will show up every year. That I won't get into the legality of it because it's the point of the law - to protect turtles - that matters. That turtles are attractive and wonderful, and it's hard to let them go, but the right thing to do. Then point her to a place she can adopt an unreleasable captive turtle, or just say they exist and need care while this one needs to stay in the wild.
Hth
(in my humble opinion).