My six year old granddaughter lives with her dad and stepmom. i just learned that he has been drag racing with?

her in the vehicle with him. I want to say something but am sure it will lead to an argument. He is a hard person to get along with, what we call in the south a red-neck. How should i handle this? I also don't want to jeopordize seeing her, but I do fear for her safety. I told her to not get in the vehicle with him anymore but she is only six and seems to be somewhat afraid of him at times. My daughter can't say anything as he and she get along pretty bad at times. please i need advice. thanks