This is a difficult situation for a 13 year old. School year starts soon. Telling a school counselor would be a start. And, sometimes simply acting directly is best. She could call the police non-emergecy number and make a report noting that she had been in foster care before. She might try to contact her old case worker if she knows who that is. The sexual abuse leads me to say she should call the police and tell them that she is a former foster kid. This is a very sad case.
Answers on Avvo are for general information purposes only and should not be relied upon as legal advice. No attorney / client relationship is created by providing this answer. For specific advice about your situation, you should consult a competent attorney of your choosing.
Another important consideration is a person's legal obligation to report this abuse. There are laws in the Texas Family Code which make it a crime to fail to report known child abuse. "A person having cause to believe that a child's physical or mental health or welfare has been adversely affected by abuse or neglect by any person shall immediately make a report as provided . . ." It is a complicated matter, but suffice it to say if you know about child abuse occurring and do not report the abuse, you could be prosecuted for a criminal offense yourself. Please beware.