What can be done about someone who posts untrue slanderous remarks about a person on an Internet site?

I know slander has to be spoken, but what do you call it when it is written? What happened is an ex-coworker of my sister made postings about her on a website. The remarks are very very malicious, and untrue. She did nothing to this man, and it could be that he wanted to be with her and she wasn't interested in him. If you type her name in Google the story shows up #1 on Google. Can he be sued?