Pisses me off when people do this who are parents. It's one thing to ruin your life...but to ruin your child's life too is unexcuseable. He needs to straighten up for his children. Have no tolerance for that crap. I wouldn't even let my children near him.
Maybe set up an intervention with the adults who love him and get him off that stuff. Apparently the part of your brain that knows that this is bad for you is the same part of the brain that the drug eats away at first. He will not know what he is doing is killing him until he is off it for awhile. If enough people get together to convince him...he might listen.