Get educated on the issues of sexual assault and harassment.
Be respectful in your support of your friend, and do not engage in any actions on his/her behalf that could be retaliatory in nature. Allow the University resolution processes to fairly and impartially adjudicate a report.
Encourage your friend to make good choices and to abide by any restrictions imposed by the University.
Be mindful of your own needs and be sure to take care of yourself. People in supporting roles can benefit from professional assistance by speaking to a counselor.