Internet!

Hi!Hope somebody can help my friend on this problem. My friend uses the internet frequently and just recently she learned that an impostor is using her name to malign her to other people.The problem is spreading like wildfire that my friend is being judged because of what this impostor has done.The impostor uses Yahoo messenger and mail,is there a way to report this so this person will be given punishment? I'm very sad for my friend because of what happened to her.It just shows we are not safe....... Thanks for answering and happy mylotting!