Hello, I'd like to help you with your question.
It can be very difficult when you see a family member in a bad relationship, especially one where they are getting hurt. And it sounds like this is what is happening with your daughter. The man she is with sounds like he is hurting her emotionally and physically.
If he has been diagnosed with Narcissistic personality disorder and Bipolar, the best medication for him will probably be Lithium which helps to stabilize his moods. Personality disorders to not typically respond to medications but the symptoms do, especially if the person has symptoms like anxiety
or depression associated with the disorder. Your daughter's boyfriend will most likely be forced into treatment because of his situation so the therapist and psychiatrist will do an evaluation to determine what treatment, including medications, will work best for him.
When trying to help your daughter, be sure to use "I" statements, such as "I feel bad when you...." It helps keep her from feeling defensive. And this allows you to express your concerns and have her hear you.
Since it sounds like they both will be living under your roof, you can set rules. Be sure to decide ahead of time what rules you want. For example, if they are living with you, they need to contribute to the income of the home. So make a rule that they give at least a certain set amount each month. And that they do certain chores, etc. That way, they know they cannot just take advantage but must earn their way.
Also, consider talking to your daughter about seeing a counselor, with or without you. She needs to talk to someone about her situation and why she continues to let this man into her life. Children do need their fathers, but the harm he is doing to her (and potentially to the kids eventually) is not outweighing the good. The kids will learn that harming someone is acceptable. And that is not good for any child.
I hope this has helped you,