Can you get fired from your job for going into substance abuse rehab?

My husband just went into substance abuse rehab. When told his boss he was admitting himself in the next day, they fired him. They have known that he has been fighting this addiction for awhile and thought he had been clean. It is a drug free workplace and I don’t know how but he just recently passed a hair drug test even after he admitted he wouldn’t pass it. What is the law on this? We live in missouri.