Be supportive, listen and be there.
If it is at all possible, stay with her at her place or let her at least spend some time at your place. This is not the time for her to be alone.
She may need to talk at odd hours, or a great deal at the beginning. She may not have a lot of people she can talk to and she may over rely on one person. Be there as much as you can and encourage her to either call a hotline or go for counselling.
She has been badly treated. She needs to be nurtured.
This is crucial. Many rape victims blame themselves. She needs to be reassured that the rapist is to blame, she is not.
Everyone has her own timetable for recovering from a rape. Do not impose one on the victim.
For example, suggest she go to a hospital and/or call the police.
Encourage her to make her own decisions. She needs to feel in control of her life and this will not be possible if you do everything for her.
Deal with them somewhere else. Although it is supportive for a rape survivor to know that others are equally upset with what happened, it does not help her if, on top of her feelings, she also has to deal with, for example, your feelings of rage and anger. If you have strong feelings, talk to another friend or a counsellor