Legal Question in Criminal Law in California

Yes, the witness can and must go to court. Failure to comply with a body attachment can result in being arrested and jailed until the person complies. That does not mean that the person cannot tell the prosecutor that they do not want to testify and that if they have to testify their testimony will not be favorable. (This will go a long way toward convincing the prosecutor to back off)

This answer presumes that the witness was personally served, if the witness was not personally served then the body attachment could be attacked.