Alcoholics DO drink because they are alcoholics. Your boyfriend drank because, although he hadn't had a drink in two years, he was not fully recovered. I know alcoholics whose young childrend died tragic deaths and they didn't drink over it.
As for his ex, she sounds like a very sick person in her own right. She obviously thinks she can manipulate him. It would be wise for him to document any threats she makes. If she has threatened to kill you, you could sign a complaint for harassment. Depending on the laws of your state, your boyfriend can probably get a restraining order, and even you might be able to get one.
Are you going to Al-Anon for yourself?