Legal Question in Criminal Law in California

The hospital is required to call the police in instances like this where a person is seriously injured in an assault. The police will forward their report to the District Attorney, who will decide whether to file charges.

If the male visitor is charged with a crime, his defense attorney will probably argue that he was protecting himself and the ex-girlfriend.

Your friend went to his ex-girlfriend's house armed with a deadly weapon. He argued with her, then he went looking for the guy, still armed with the baseball bat. It would seem reasonable for someone to take steps to incapacitate him.

The question will be whether hitting him with a 2x4 would be reasonable under the circumstances. Since your buddy had a bat, it seems like a pretty even match to me.

If your friend forced his way into the house, his ex-girlfriend is presumed to be in fear of her life, which would authorize use of deadly force.

I have to agree with Mr. Marshall. You are going to have a long search for a jury that would be sympathetic to your friend, after he was stalking his EX-girlfriend, was aggressive with her, pushed into her home when he was not welcome (which is obvious if she had another man there), and then went looking for the man with a weapon. I can't imagine how your friend can convince either a jury or himself that he is the victim. He was the assailant. He just wasn't as good at it as he now wishes. Your friend can forget about suing. I can't imagine a lawyer taking his case on a contingency basis.