Honestly, I would sit back and have yourself a good little laugh. I've had neighbors around 8 years old so excited to ride their Grandma's horse next door, and then realize it comes with a price. "We have to pick up POOP?!" Also, I've had the overconfident boyfriend thinking riding horses is easy and watched him get dumped on the ground 3 times in under 20 minutes. As long as she doesn't get majorly injured, just have a good laugh. Just remember that you warned them, so when they come back whining, you can say I told you so.
But then again, I am a rude teenager that gets extreme humor over other people's ignorance. Especially with horses... "Whats a sheath?... OH EWWW!"