A.,
Thank goodness for people like you in this world. Even though Andy is not your son, there is a moral responsibility to help protect someone in danger.
I am assuming you've tried talking to your friend and she's not willing to change anything? What about talking to her mom? What about an intervention with all her family and friends (even if it's a few people) to talk to her about her drinking, depression and treatment of her son? Maybe her mom should protect her grandson and get the mom "Kathy" out of there; is she in tune with what's going on? I know that seems drastic but some people need "wake up calls" or need to "rock bottom" before changing. I know this from personal experiences.
If nothing works, you will need to call CPS. They are not the say all, end all and do not necessarily do anything but it's worth a try and if you get a good social worker, they may help Andy. You can only do what you can do. You have a 3 year old to take care of.
I would also reach out to any organization that can help. AA, churches, etc. Don't give up. A person's life is at stake and more importantly (to me) a child is at risk.
I fully believe most of what's wrong in this world is bad parenting. No other way to put it, bad parenting.
You're an angel. Hang in there A..
God bless you.