She can and should report any physical or sexual abuse to her local police department. As for the custody issue that is more a family law question and I have added tags for child abuse, child custody, and family law so that a lawyer in that field can better answer that question.
My answering your question does not establish an attorney-client relationship. In all legal matters you should make an appointment to sit down with an attorney to discuss any and all facts that may pertain to your unique situation.
As to the specific family law question of whether a 15-year-old can express a preference to the Court as to where she would like to live the answer is yes. However, that does not mean the Court will follow her preference. The Court will make all decisions by determining what it believes is "in the best interests" of your friend - including where she lives. However, it is important that she expresses to CPS, police, Courts, etc. why she believes a particular place is the best place for her.
This communication does NOT establish an attorney-client relationship. This answer is intended for informational purposes only.
She should not be afraid to tell the truth. Growing up with abuse can lead to a life time of difficulties. I'm glad she's been brave enough to report abuse. She needs to continue with her acts of bravery.
If her father wants her, he can file a suit to modify the parent-child relationship and ask the judge to interview your friend privately in the judge's office. The judge will do that. The judge is not bound by the child's preferences, but between the child's preferences and the abuse allegations, she has a good chance of going to live with her father.
If her father does not want her, then it's almost impossible for anyone else to file a lawsuit to take her out of that situation. Look at Texas Family Code sections 102.003 and 102.004. If the state files a lawsuit to remove her, anyone over the age of 21 who cares about her could file an intervention and try to become her temporary custodian or foster parent.
Please encourage her to stay strong. My heart breaks for her having to endure such unforgivable circumstances. Your strength and friendship is a blessing in her life.
All the best...
Where is her real dad? She should ask him to file a modification so she can live with him. Second choice is that she should continue speaking up and telling the truth to CPS, police, teaches, etc.
Be her friend and let her talk about all the conflicting things she us feeling.
This answer DOES NOT establish an attorney-client relationship. This answer is based on the limited information provided and is not intended to be conclusive advice. There are likely other factors that might influence or change the advice after a more lengthy consultation.