Unstable employee

I am a supervisor in a call center enviorment. I have an employee who has become very mental unstable in the past year.. He is taking several medications and has been known to call other employess after work hours completly out of it and then does not recall even calling and speaking to someone. He has even called into the office then states that he does not recall doing so. It was suggested that he use some FLMA as he was not coming into work, he was not doing his job very well at all. As of recent he was sent home because he was clearly highly medicatied and not able to do his job. The following day he called in to the office several times asking to borrow money from anyone that would answer. One supervisor feeling sorry for him said that she would lend him the money. Then he called to say that he would not be in that day because he had hurt his foot. He however did arrive at the office to get the money that was offered to me leant to him. He arrived with a gentalmen that could be heard in a threating tone he better get the f@$kin money. The supervisor became nervous went back into the building and asked another male employee to join her outside. There have been several documented issues in this regard. He was on a final warning when he went on FLMA. When he returned about 1 month ago he was put on another supervisors team. Whom has repeatly seen the same issues I was having with this employee behaving comepletely out of the norm for this employee ( until this last year) The employer now has asked that he be put on a second final warning. Stating that they want to protect the company. However there are several employees as well as supervisors who are now scared working with him and his mental instablity. While he has never been violent to anyone as of yet. He actions have changed so much after the last year and get worse each time, that other employees are in fear that he will come to work and hurt someone or bring someone with him that will. As employees do we have any rights? Anything that we can do? We all feel as though we are working in an unsafe enviorment. Our job is stressfull as it is now with this additional issue it is creating more undo stress.