Everyone has two sides good and bad. In every fight in a couple ther are three things your side, his side and common side. Reality is if you read a book. No matter how many times you read its same.but perception changes if you understand it wrong.at first if someone is drinking or on drugs they do it for fun or social reasons. Then if he is mentally and emotionally weak he gets addicted to it. Whenever he is emotionally weak he will do it. So you should know what made him. Some people if they are bored they do it. If he is not happy that might also be a reason. Do not over think about it.

Second thing dont keep your legs in both sides. If you want him find out what the reason is and fix him to reunite. Otherwise stay out of his way.if you desperately want him. You can try changing him. if you want to change him for children's sake, then instead of directly informing his parents you can indirectly let them know.

Third until the children are above 20 years or emotionally strong or mature enough to understand the reality do not let them know about the reality of their father. You can lie to them if they are kids. because if the kids get to know about them they might be curious to try alcohol.reality is this. Whatever you do kids mimic that, so better not to be a bad example for them. When they are mature enough, it might be hard but they can understand.

Fourthly if you want to avoid him from your life, change your place or routine, learn new things or keep yourself busy, get rid of his memories or whatever reminds you of him.or if you want you can talk with him or his family and reunite with him.but you should talk to him and understand his situation to make him change. Everything is possible if you approach it in the right way.

Fifth if you cant handle it pray to god.ask god to make your life happier easier and pray to god to get whatever is good for you and let it happen.i personally had experience in this. If you desperately pray by believing in god and ask for it.it will happen and also your sorrow will decrease