You can report it to Tumblr: Tumblr
They can be reached at abuse@tumblr.com

They may follow up by contacting the authorities where she lives. At the very least they would most likely suspend the accounts of the bullies and remove their posts.

If you have any idea what area she lives in, you could try contacting the police there.

Horrible people, those bullies. Don't they have something better to do?