My fiance works in a warehouse where a new manager took over and mandated no heat to be turned on in the warehouse. (What is ironic is that the manager and other supervisors who have offices are using electric heaters to keep warm!) Is there anything one can legally do about this situation? It is extremely cold and the employees have to wear coats and gloves to stay warm. My thinking is that if my fiance gets sick or frostbitten from the cold, this could be a workers comp claim. The manager just said that's why they have medical insurance benefits.

You can double check with OSHA but many employees are forced by the nature of their jobs to work in extreme cold or extreme heat positions; I don't believe there is any law about the temperature of the workplace. I could be wrong, though, and OSHA is the place to find out.