Labor laws...Is this legal?

Can a diabetic (insulin dependant) be fired from thier job for having episodes of very low blood sugar while at work?

even if a diabetic can be considered disabled or handicapped and fall under ADA they could be terminated just not for that reason alone. all a business has to do is "make reasonable accommodation" for a handicapped employee. if an illness prevents the basic duties of a job from being performed an employee can be terminated for poor job performance or excessive absenteeism or any number of other reasons. they would not be fired for having diabetes but instead for failing to perform their duties. business are expected to make an effort to employ those with disabilities but they are not expected to hire someone who with assistance or modifications still can not do the job. the same is true for an employee without a disability if their health prevents them from doing their job then they can be terminated. any good company would make an effort to work around a health issue and would try to find a way to improve the employees performance. if health issues can not be resolved or improved then in many instances employers have no choice but to terminate employment.
since you did not state what type of employment there could also be the issue of safety for the employee with diabetes and for others which could result in termination.

depends on the type work you do, having a diabetic spell can cause one to pass out, and if you were driving an 18 wheeler this could be very chaotic, This is just one example. You may want to contact your local labor board or a lawyer for the best answer