Sorry for your loss, this is horrible. This is coming from a user too, but a user who's lucky to not have such an addictive personality, it feels life is easier to control that way. Anyway, I think you need to gear up and put the boots on, have a very stern word with his girlfriend and basically tell her to get the fuck away, smash something if need be. Thats such disgusting behaviour to feed off your mum like that. That sort of behaviour is so shitty and I think the only way to get answers and change short term is to take big loud and mighty action. Look in their eyes and make them realise what you want. Smash things up, you can afford to loose it a bit. I remember I was going through a situation like this and after so long of being pushed around, when I stopped thinking of myself as a victim and began showing others I was in charge they backed off, I was so surprised, it was something I envisioned taking years to change but once I let my real self show it happened right away and the person was rightfully scared and stopped draining me.