My friend has been having a recurring nightmare every night for the last three weeks that involves the suicide of someone. Its preventing her from sleeping at night and I am really worried about her well-being.
What can she do to stop the nightmares? Is there anyone she should see for help?

Does she have any issues in her waking life? Like depression or anything like that? Does she know who it is she is dreaming about? If so, does that person have any issues that could result in self harm or suicide or anything?

She doesn't have any issues herself, but the person she is dreaming about does have depression. I think she is worried that he will kill himself. She is not close to this person however.

Would going to a counsellor be a good idea?

Talking to someone about your issues, even nightmares, is always better than bottling it up. Though it is probably her friend who is the one who really needs help, but that depends on her friends willingness to be helped, as I have found that in my personal experience, depression is more likely to be beaten if the depressee actually decides to get the help and is determined to defeat it.