I agree couples do need to work things out for themselves, but I also think when you are in a relationship that is parent and child, other factors are brought into play. Hi, 3Angels. My heart goes out to you. I can see you are concerned, and understandably so. If you see your child driving off a cliff, it's hard to say, "Well, they'll just have to learn". There's a responsibility to say what you see and what you feel and believe - this is what good friends do for one another, and I don't see where being a parent means doing any less - while also being aware that you can't jump in the car and take the wheel yourself.
It's so hard to watch your child in a destructive relationship, and this is a very dangerous situation. Not only in the harm his bipolar girlfriend can do to him personally, but what could happen to him through the legal process if he should be accused of physical abuse - especially by a petite 5 foot girl. It's good that so far she has told the truth to the police, except for her physical rages, and that could possibly be a problem later down the line.
I don't believe it's a matter of having confidence or not having confidence in your child. It's a matter of seeing a potentially dangerous situation beyond what has already occurred. Information is very important. You can't force it on another, but you can let them know it is available. You can let your son know you're here for him, as it sounds that you have. And you can give yourself the support you need. That's what this community is for. Thank goodness for those of us who have found this place and no longer have to figure things out totally on our own. This kind of behavior is so crazy-making and it's so easy to get sucked up into it and blinded to behavior that every one else can see, that it really helps to have someone else bring to light the whirlpool of dysfunction that's pulling us under. I doubt that I would have ever figured it out myself. I needed someone else to help me connect the dots, but I, also, needed to be willing to see the picture that unfolded before me.
No one can make you see. You can't make your son see, and since he won't discuss it with you, all you can do is decide what kind of boundaries you want in your own home. I find it alarming she has gone off her meds. It's totally within your right to tell your son, based on her history, you are concerned over your own personal safety and the safety of your home, and you can state she is not allowed in your home under those circumstances. You have to decide what is best for you. While you can't make decisions for your son, you can make them for yourself.
You can, also, periodically remind your son, that you are here for him to talk to - not as a professional, but as a mom. It sounds like this is what you are doing. You can point him in the right professional direction if he is interested, but it doesn't stop there. Parents don't stop being parents just because a child reaches a certain age. Certainly the dynamics and rules of relating change, but the relationship does not. Parents can and should be valuable resources for their family for the rest of their lives. We've replaced parents and elders with experts and professionals, and in my opinion, that's part of the reason for rampant ageism in our culture and disregard of the older ones...but that's another rant.
And when you've done what you can, then step back and let yourself be nurtured in your own faith, however that may be for you, and trust the process. It's a dance - to take action and to let go, and it's normal to trip over your own feet a few times.
Keeping you in my thoughts.