Is there a legal document I need to fill out for a non prosecution affidavit in Texas??

My son's ex girlfriend claims she was on her medication and didn't realize what she was doing when she told a police officer that he had beaten her up after an argument. They did argue (in a public place) and it was witnessed but only for yelling at one another, no one saw any physical contact. He was arrested and she has said all along that he didn't hit her after the fact. She had only said it because of her medication and being mad at him. She's willing to sign an affidavit but is afraid of being prosecuted. Either way, she won't show up for trial and they broke up so we don't even know where she is anymore. We could probably find her to get the affidavit but we want to make sure it's worded such that the court/prosecutors will accept it. In addition, she has been arrested many times but my son has never been in trouble. In fact, had the police officer bothered to check, he would have found that she had an open warrant at the time of the incident.