My new girlfriend was raped by her ex, what can I do to help her?

Let her lead the way. Hopefully she has already gotten professional help. If not she should. Be very thoughtful and understanding when it comes to any type of sexual activity. Let her lead the way. Get informed about rape victims and how their lives are affected. Be honest about the fact you can't completely understand the full effects on her life and emotions of the rape. Tell her you will help her any way you can.
Two things. She needs to be in counseling for her own particular issues about this past event. And the two of you should be in coouples therapy to discuss how the two of you should work together on this past issue, and on your current relationship together. She may or may not want to share with you all the details of the past right now. But you two need to work together on the new relationship together.