If an employee is required by safety or health standards to wear a hat during work but because of religious reasons cannot wear a head covering, you could try several approaches.
You could explain to the employee that due to state or federal safety and health laws — which constitute a business reason — the employee cannot continue the job without that hat.
Or, you could ask that the employee go to his or her religious leader and ask for an exemption from the rule barring hats for business purposes.
If, after these efforts have failed, the employee will still not wear the hat, then perhaps you can give the employee something else to do that would not conflict with the his or her religious beliefs. Termination should be a last resort.