I'm in a very, very similar situation, my girlfriend is 20, I'm 23. After making a complete jerk of myself attempting to remedy the situation, I realized that there is no real way for my girlfriend to completely disregard her mother's demands until she is "out from under her," meaning that she still relies on her for support and still feels bound to her, despite a completely appalling track record (bailing on H.S. Graduation, threatening to withhold financial support while in college, etc). Your description of your son's girlfriend's mom fits my situation to a T. It is going to very frustrating for your son, but you're both going to have to realize that she (the girlfriend) will eventually distance herself from her mom and hopefully mom will learn to not interfere. Otherwise, let sleeping dogs lie and trust your son's girlfriend's abilities to deal with mom. Hope this helps.