Hi, I'd like to help you with your question.
Sometimes people feel that this type of action during sex is ok for them. Usually though, it indicates a past history of abuse of some sort, not always necessarily sexual. People who want to be hurt in anyway including sexually feel they deserve to be hurt. This is may be how your friend coped with the rape. She turned it in to herself and decided that it was ok to be hurt so she could cope with the pain.
You do not have to do anything sexually that you do not feel totally comfortable with. If you do not want to do this, then talk with your partner. Let her know it makes you uncomfortable. Do this in a non judgmental and gentle way.
Depending on your level of openness with each other, you can also gently introduce the topic of her rape and ask her if she feels her sexual preferences related. Do this only if you feel it is ok and if she has brought the topic up before with you. She may or may not be aware of her reactions so tread softly here.
If you feel she is in pain and needs help, offer to help her find a counselor. Here is a link to help you:
Here is another link to help you help her:
This organization can give you ways to help your friend recover from the trauma of the rape and begin healing.
I hope this has helped you,