The wrong crowd

Hi my house mate of ten years is having the worse period I have ever known , He is 40 this year and I don't know whether that is a factor for his behaviour . In the last two years he has fallen in with people from hostels who have really just taken advantage of him in every way money wise . For my stupidity and because i care for him so much I have given him a lot of my own money which I don't have much i work full time but I'm silly enough to believe him when he says he needs money for certain things etc . But then i find out he is giving it away to these guys for drink and drugs . He only really bothers with one now which is a female and because of his aspergers he feels that women has always rejected him because of this and this girl giving him attention has made him fall for her . The only thing is she has a huge drink and drugs addiction which is heroin me and his mother who is in her 70s have both tried to get him away from her but with no luck . We have no support from anyone in our area we have tried everything but every time we try and get him away from her he loses his temper so much it is scary . This girl is bleeding him dry and she is trouble I had a guy outside my house the other day and I heard him threaten my housemate when i run out he was shaking with fear . I threatened this guy and nearly lost my cool but warned the guy off but we live in a lovely area with great neighbours and fear all this attention is gonna turn them against us . Please any advice on how we can get him away from people who just take advantage of him for his money etc because we really don't know where to turn and are scared he will be either beaten by these crowds or also get on the heroin himself . Any advice would be great thank you !