Well coming from someone in your shoes you absolutely need to get custody. Her sole purpose in the short term is going to be maintaining this habit. She can’t sort herself out unless she admits a problem and most definately can’t do it without intervention from professionals. From your point of view your responsibility is your son. She can only be responsible for herself right now. You have to sell the idea it’s best for your son and her if he’s at home. That her mum won’t figure it put if he stays with you etc.
I know exactly how you feel, I have 3 kids and an addict husband. I’ve just had to remove an entire overdraft to stop it from ending up spent and now having to do various weird things to safeguard property, vehicles and possessions. It’s madness. It never stops. I just do what I have to to keep family together and life as normal as possible. I try to keep family life and his antics separate so that family life goes on regardless and if he does silly stuff it doesn’t affect us too much. I call it damage limiting. You having your son limits damage to all of you, you can care for and distract him while his mum either gets help or self destruct. Either way he won’t be as affected than if he was watching it all unfold first hand.
Hope this helps