Abused aggressive dog...help!!!

Hello! I have been friends with someone for about 15 years and for the last 12 years, I have known her dog, Geronimo. We knew him as a puppy becuase he was born at my friend's boss's barn. While he was there he was very abused. My friend then took him from there, but her own life situation at the time was very chaotic and she herself comes from a very abusive background, which of course led to her having anger issues of her own. She had the best of intentions when taking the dog, but the environment he was in was still an unstable one, although not abusive towards him, I am sure that he picked up on all of the negativity around him which only compounded his problems. Some of the people around him....friends of her siblings especially...would laugh at him and provoke his outbursts of anger. Then eventually she took him out of that house completely, but also got another dog who is large and very 'in your face' which I think triggered many memories of abuse. I have known and been around Geronimo mostly his whole 12 years of life and I know how sweet he can be, but it's as if something will trigger a switch to go off in his brain and then he becomes vicious and it seems like he doesn't have control over his emotions or actions at that point. Recently my friend, his owner, found out that she was pregnant and realized that she needed to find some way to try and make it so that it was a safe environment for everyone in the house. Again, her intentions are good, but there is too much chaos with the other dogs in the house and the hormonal outbursts of my pregnant friend for any progress to be made there. Geronimo has been medicated in the past for his issues, half a human dose of prozac a day for a 15 lb. Jack Russel Terrier, and when that didn't work, the vet told her she should just put him down. Despite how vicious he can be at times, putting the dog down is not something she wants to do at all. So....I took Geronimo in about 3 days ago. He is doing well with me, I believe because I have the ability to remove much of the chaos and stressors from his environment which would normally cause him to 'freak out'. BUT...I have a cockapoo puppy as well and she is very playful. She spends the day at my parents house while I am at work and she plays with their dog, Harvard all day long. My puppy loves to play rough...growling and biting, but not out of anger. Harvard tolerates everything from her, I think because he sees her as a little sister and he understands she is just a puppy. That becomes a problem, however, because Geronimo feels threatened by just about everything, so when she growls and nips at him, thinking she is playing, that switch goes off in his brain and he becomes very angry. I researched homeopathic remedies and thought that straphysagia seemed like the right way to go for his problems, but I would like to know if my conclusion is right. I love Geronimo, and always have, and i don't want to see him put down for something he has no control over. I can see the differnce in his eyes when that switch goes off...that is not him and he doesn't know what he is doing. When he is being 'himself' he is such a lover. He curls up right next to me and wants nothing but love, but then something will startle him, or someone or something will get too close and trigger memories of so many years of abuse and that can become dangerous. Please help me figure out a way to help this dog!!!!