You sound like a true friend and I'm happy to suggest some things you could do to help your friend.
First of all, she could be using the opiates to self-medicate her schizophrenia symptoms. This is called MICA: Mental Illness/Chemical Abuse and is best treated by a two-pronged approach: treating the drug addiction at the same time the mental illness is treated.
Yes, I do believe schizophrenia can be triggered by stress or by abuse and also, it can be caused by drug use in a significant number of cases where people are heavy drug users. Statistics bear this out.
If your friend trusts you, I suggest you offer to take her to a professional who is in the position to diagnosis or rule out the schizophrenia. She may not quit the opiates right away, and that is the nature of drug abuse: more than one relapse is possible before she gets clean. So she needs to get medication for her schizophrenia even if she is still using drugs. Continuing to use drugs will hamper if not outright prevent her recovery from the mental illness.
Again, you sound like a great guy and you have compassion and wouldn't judge her for the choices she made (to use drugs) and you are also concerned because she was abused. Stand by her side. Develop a relationship of trust. The Xavier Amador book I refer to often for friends and loved ones of people who resist treatment is called I Am Not Sick, I Don't Need Help. He encourages the friend or family member to use a technique called motivational interviewing, whereby they develop a relationship of trust with the person with schizophrenia, and gently inquire about their life goals, because everyone has a goal he or she wants to achieve. Once the trust is built, you ask if you could suggest something that might help her obtain the goal, and your recommendation would then be medication for her schizophrenia and that she stay in treatment. The technique of motivational interviewing was indeed originally used with people recovering from substance abuse.
James, I personally know people who recovered from heroin abuse, and this is entirely possible too, as well as recovery from schizophrenia.
So lastly I want to say thank you for being such a good friend and staying with her through thick and thin. Your ongoing friendship could be the thing that turns her around. Too many people abandon friends who develop schizophrenia.