While I agree with Wingin'it that her bipolar disorder should not excuse her behavior, it does sound like the people who are supposed to be treating her are failing her. Even though her CPN probably can't give you information without her consent, YOU can give her CPN information. Call the CPN and tell her that your girlfriend seems manic and is exhibiting dangerous behavior. (Cheating is one thing...bringing home strangers is another thing altogether and in my mind, with everything else going on, would be grounds for hospitalization.) The CPN may not be getting the whole story - your girlfriend probably doesn't see her behavior as unusual and just tells the CPN she's "fine".

Certainly, don't let this go on indefinitely and end up hurting your own mental health. If the relationship needs to end, then it needs to end. But I would first take this to the next level and intervene by getting in touch with her CPN.