In Texas, a witness is subject to criminal penalties for not showing up if they have been subpoenaed. However, the reasons for your friend not wanting to attend do seem reasonable. The best thing to do would be to call the District Attorney's Office and explain her fears and concerns. The only problem is that often times the only witness in a domestic abuse situation is the actual victim. If she doesn't show up, and she is the only witness, the defendant has a better shot at winning the case, especially if there is no other witnesses to the case. Sometimes, if a witness is unavailable, the court is allowed to introduce into evidence the 9-11 call. There are constitutional issues with this however and thus that is why the DA's office usually needs/wants the witness to testify in court.
Unless the party that sent the subpoena relents, your friend will need to appear. I would contact the DA and explain the situation. Perhaps they can work out details so the your friend feel secure.
Specifically, she should ask the D.A. to petition the judge to allow her to appear and testify without giving her address, current job, phone number, or any other identifying information that could help him find her. Based upon past threats and batteries against you, she could also probably prevail in getting a restraining order barring him from ever coming anywhere near her, contacting you or harassing her. She should also know that if she has such a restraining order in effect, it is a defense to the crime of carrying a concealed firearm without a concealed weapons permit under Penal Code Section 12025.5. Hope this information helps.
In addition to the answers already provided, your friend should know that in California, a witness must be personally served with the subpoena or the subpoena is invalid, i.e. she need not appear. If your friend received the subpoena in mail, or if it was left with someone else, she is NOT required to appear in court. HOWEVER, if she contacted the prosecutor that served her and acknowledged receipt of the subpoena, and agreed to appear in court, then she has "accepted" service of process and will be required to appear. Failure to appear could result in contempt of court.