Health Parameters in workplace

I have an employee that has a very unfortunate medical history and work history.  This employee has sued her last 2 employers for various things (COBRA violations, etc.) and tells me that she keeps 3 attorneys on retainer at all times just in case she needs one. According to her she regularly sues people. I have a good relationship with her, but believe caution is always required when dealing with her.  She is salaried and has been an employee for 2 1/2 years.  Almost immediately after coming to work for us she was diagnosed with metastatic breast cancer and had to take off work for for several weeks due to surgery and chemo.  She had an inordinate amount of time off for follow-up doc visits, chemo, etc.  She has since shared her medical history with me and relates a history of two prior strokes.  She currently sees her doctor regularly for uncontrolled high blood pressure.  2 weeks ago, while I was out of the office, she left work due to a very high blood pressure reading.  She went to see her cardiologist who sent her to the emergency room.  At the ER she relates that she was diagnosed as having a TIA (transient ischemic attack)and temporarily lost her vision in her left eye.  This was on a Friday.  She returned to work on Monday, said she was better, that her vision had returned and that she would be seeing another specialist regarding her blood pressure that is still uncontrolled.  My question is this:  In my employee handbook, we require an employee to provide proof of illness from a physician after three days of absence" - this is the only illusion we make to a physician's release to work in our P&Ps.  With that in mind, would it be reasonable for me to ask for her for a letter from her physician releasing her to work and more importantly what parameters her blood pressure must within to be to be allowed to work?  I realize she has already been back to work for 2 weeks, but am more concerned about her continuing to work when her blood pressure is still too high.  We are a medical facility and are very consicious of medical condidtions.  I fear she may be working when she shouldn't be as her blood pressure is still high. I don't want her having a stroke while she's at work and I believe she is a walking time bomb. I also want to be sure I'm not requiring something of her that would put us in jeopardy of being sued.