What to do about a co-worker who displays threatening behavior?

My aunt's work enviroment has become very stressful. On occassion she has felt physically threatened by a co-worker.

This person has been even bellegerent to her in front of her boss. When this happened, it was the first time she said felt this person might be capable of being dangerous. Previous co-workers have also said that they felt she could be dangerous.

Due to the economy it is very difficult for my aunt to find another job, but I do not feel this is appropriate environment for her to work in.

Has anyone else been in a similar situation and how did you deal/ resolve the issue.