BELIEVE your friend.

LISTEN without judging.

TELL them you care. 

FOLLOW your friend's lead. Support his/her decisions.

REASSURE your friend that this is not their fault.

RESPECT your friend’s confidentiality; do not confront the attacker.

EXPECT all sorts of emotions, including lack of emotion and/or shock.

GIVE your friend time.  The recovery process is different for each victim and it may take time.