I agree, she should have known that was not funny. She had no idea how the dogs (or anyone: dog or human) would react. They could have easily "attacked" the 4-wheeler, causing her to have a crash. If the leashes had gotten into the drive chain and spocket, she would probably have been pitched over the handbars head first.

Try and talk to her about that being not a nice thing to do. She could have caused injury (maybe serious injury) to the dogs, you, and herself. I doubt she had a helmet on, and I doubt she was fully clothed. When we allow our son (he is 13) to ride a motorcycle, wearing a helmet and being fully clothed are requirements.