If you don't have any further discussion with her son, I suggest you at least let him know that you know she should not have been driving with him in the car, that it could be VERY dangerous to ride with her in that condition, and ask him if there is someone he could call (you, if you are comfortable in that role, otherwise, his dad or another trusted adult) if she tries to make him ride with her when she has been drinking. If he is interested, he would probably benefit from Alateen, where he could talk to other kids in similar situations.
My best recommendation for YOU is Al-Anon. I can tell you with the attitude she is displaying right now, she isn't going to get any better, and could very well become worse--MUCH worse--as time goes on.