Call her parents. Are they not capable of handling this situation?
The best thing your daughter can do is be an ally. Research points out that this is the best way to stop bullying. Actually having a peer say, "Stop," is the most effective thing she can do.
But, this should not be only up to your daughter. The principal should be called and this should be handled ASAP. Around here, these kinds of threats are taken very seriously and the police would be called in many situations. The fact that this has gone on for months is not acceptable. Don't you work in a school? Maybe you can help the parents understand how to navigate the system.