You could also mention that if something were to happen or she got pulled over she could be sited or charged with child endangerment so you're just trying to protect them both.

I would mention that most states require rear facing until at least the age of one, some the age of two, and it's recommended as long as possible! The car seat lady site has a lot of great information about the safety of extended rear facing. Maybe preface it with "I'm definitely not trying to impede on your rights as a mother or your parenting skills but I'm genuinely trying to help out with the safety of your child" or something like that. Don't be afraid of the confrontation. You could save a life <3