If he was in treatment then he was drinking before you ever met him..... who was to blame then???? Her??? Did she ever put the responsibility back on him?? That's where it belongs. It doesn't matter who is in his life or what he is doing in the way of career or if the sky is purple one day.... it is ulitmately his choice to pick up and down that drink!!!!!
As it was said above, it is misplaced blame and anger for this disease. You and he know the trueth and it is up to him whether or not he is going to get help and stay sober or continue the downward spiral of alcoholism. The only thing you can do is support him if you wish too and suggest his mother go to alanon.