First, although you know your son the best, don't let his age fool you. I have personally witnessed children that age removing their seatbelts.
It is so hard to deal with the stuff that involves or affects the kids. You will probably have a lot of anger in this area. All you can really do about it, unless she is being blantantly abusive to him, is use what you see to improve yourself as a father. Believe me, we all have room to improve.
"Acceptance doesn't mean resignation. It means understanding that something is what it is and there's got to be a way through it."--Michael J. Fox
