I'm not a lawyer, but I've seen male friends through abusive situations. I can't substitute for a lawyer, but I can give you some basic advice. This is a bit challenging, because as a man he's at a disadvantage in the legal system. Everything he says will be taken with a grain of salt from the beginning, and having a mental health diagnosis will unfortunately make that worse.

The first thing he needs is a good lawyer. Due to the sex disadvantage, he's going to face an uphill battle regardless of circumstances. It's nearly impossible to successfully navigate the family court system without one.

The second thing is that he needs documentation. The more credible he can get, the better. Medical documentation if she's injured him can help. Witness statements can help as well, as long as those come from witnesses whose credibility cannot be broken down in court.

If he can do so without her finding it and becoming angry about it, he should keep a journal documenting the abuse, as well. Even though his word will be doubted due to his mental illness, regular notation of incidents and their results will help to back him up in court.

In addition, he should contact domestic abuse advocates with his information, describe the abuse, and ask for assistance. Local organizations might help him, but I recommend checking out http://www.saveservices.org/ first. He can get a restraining order against her to help prevent further abuse. He should file for that as soon as possible. He may be able to force her to leave the family home once he has that, depending on what state the couple is from.

To help him, you can also find father's rights organizations in your area, if there are any. A google search for "father's rights" and your state, county, or city, should bring up several listings for attorneys, and among those listings you may find listings for advocacy groups, as well.