You just never know about someone. you know?

I just found out that a friend of mine has been estranged from his entire family for the past 23 years. You'd never know it from the way this guy seems. He has been dating my sister-in-law for the past 4 months, and everything about him seems normal. A week ago he went into a deep depression. Took a leave of absence from his job and then holed himself up in his apartment, not answering phone calls, not answering his door. This without explanation to anyone. I saw him, finally, leaving a liquor store. I followed him to his door and confronted him. He let me in and told me his story. It seems he had some severe mental breakdown 25 years ago. A painful situation that caused him to just leave everybody and everything. He became a drug addict and alcoholic, living on the streets during the winter, sleeping on trains and missions. years passed and he finally got clean and sober. Found a decent job and a decent apartment. This 7 years ago. About the time I met him. He recently found out via the internet that two of his beloved sisters had passed away a few years ago. This caused his relapse into booze and his deep depression. He is planning to get help, but he doesn't know when. He wanted to make me swear to not tell anyone about all of this. I told him that my sister-law is devastated over his unexplained behavior, so for her sake I have to tell her what is going on. He got mad at me, started to guzzle his pint of liquor and then ordered me to "get the fuck out of my house". He is my friend and he needs help. I will help him as much as I can, but what can I do if won't help himself? What would be the "normal" thing to do here?