Is it legal to fire someone because of mental illness,& for the boss to tell someone you are going to be fired

My dad has been at a company for over twenty five years and has been severly depressed for past three. He has been trying to get help, he completed an outpaitent program about two months ago. He is trying to get into an inpaitent program right now but is running into problems with insurance paying for it. Anyways my sister knows his boss and his boss mentioned to her that he was going to get fired. I was wondering if it was wrong for him to tell my sister and also is it illegal since he is trying to get help? Please i will take any advice i can get. thanks!