Wow, that's a hard situation, Vulturine. I think you handled it was well as could be expected. It was very good of you to meet with her and hopefully it will help her process her emotions and move towards healing.
There's a certain kind of loneliness in addiction. Everyone thinks at one point or another that no one truly understands them, but addicts feel this acutely. It's really impossible for someone that hasn't struggled with the problem to know what it's like. When I drank I wouldn't let anything or anyone get in the way of getting drunk. It had nothing to do with them and everything to do with me and my problem with booze.
I bring that up only to point out that your friends mom is in a very difficult spot. Bad enough losing her son but she can't understand how he came to that point. Perhaps she blames herself- parents often seem to feel their children's problems are the parent's fault. Sadly addiction just is, there's probably little she could have done to make him a different person.