Which Law Covers The Use Of Medical Devices In the Workplace?

My girlfriend has a heart condition and needs to wear a heart monitor for few days. Her employer is giving her the runaround on some arbitrary approval process. I would assume they cannot tell her no, but they are requesting she submit a written request and I want to make sure it is air tight. The problem is that I cannot seem to find any lead or regulations surrounding this. If anybody could please point me to a specific law she can reference in the letter it would be appreciated.

Edit: I am in the United States

Edit 2: I should note that she works on Tribal land, but does not live on tribal land nor is she a member of the tribe.