I think my father is having a mental breakdown. Help?

[Meredith]

My mother and father separated six years ago, after a thirteen year "relationship." They never loved each other, but they loved me, and they were too poor to think about separating. Now, six years from now, my father is living with his mother/my grandmother, and I am living with my mother. I have no siblings that live with me.

Three weeks ago my mother took a trip to somewhere in Texas (I don't want to divulge as to what city) under the alias that she would be going to one of her favorite bands concerts. When she came back, she told me that was a ruse to protect me, and instead that she went to see a new boyfriend she met on Twitter. She still hadn't told our relatives and my father, for fear that if the relationship didn't work, they might get to like the guy, and then it would be over.

She accidentally told them through Facebook, and now my relatives feel a little odd over it, like me. I don't really know what to think; after six years, my mother accidentally found a boyfriend? She wasn't planning on meeting anyone.

But, that's not the problem. The problem is that I seriously think my father is having a breakdown. He smokes (cigarettes and marijuana) an drinks heavy liquor, and he has almost no friends. That is bad enough, plus with his anger streak and horrible memory, he really thinks my mother cannot be trusted, and that she used him for money years ago, to pass through college and get her graduates degree. Now my grandmother says he talks to himself, smokes and drinks more than ever, and he keeps murmuring to himself about how my mother is "going to run off to Texas". He even listened in on a phone conversation between my mother and my grandma, standing in the dark, by himself. I had a recent conversation with him over the phone, about how I feel. (in very frank terms. I know, not a good plan, but I wanted to get it all out as soon as possible, like ripping off a band-aid.) After that, he "gifted" me on Steam with the new Elder Scrolls game, Skyrim. Then he said that he was an a hole and that he's sorry he hurt me so much. I'm seriously starting to think that he's going insane! I don't know what to do, tell the authorities and get him sent to a rehab or mental house, when I know those aren't good places; or just let him live with my grandmother and scare her to death every day! What should I do?