If she did not have his permission to read his texts and/or to record or listen in on his conversations, she committed a crime commonly known as "wiretapping" and could potentially be prosecuted; in this event, therefore, you could contact the police to file a report and look to press charges. (Though even if she did not have permission at the time, if he now tries to protect her by claiming she did, or generally does not cooperate in the matter, the case will likely not go anywhere; as a practical matter, his cooperation in the case is critical.)If he did give her permission, she did nothing legally wrong: one participant in a conversation can legally let another person record it or access/read/etc. his texts. So if he consented to this, while it was creepy and immoral, it is legal, due to his right to let her in on the conversations, etc.