I can't imagine the frustration and pain you must feel seeing this and want to help.
As the others have said on this thread it is so important to take care of yourself by perhaps going to these meetings. I do have concern for you saying that you would rather see this person do the drugs in your home, yes I have seen that on intervention too. But have you also seen the frustration of the family still not able to control them and they still could die. Praying to God is a great thing to continue on with because most everyone that i have talked to who have been able to get into recovery and stay sober have had a spiritual intervention from God.
Addicts are very smart manipulative people and go to any length to get high ( I know I am a recovering addict).
This person was in detox for a year or a recovery home, either way it is very hard work, but it can be done. As you can see with the show intervention, its not really tough love but are at the same point that you are at.. getting to the end of ourselves and perhaps the people at AlAnon can direct you now with what you can do, and support you if he continues on.
One good thing is that he has been in the program before, so in the back of his head he does know all he has to do is reach out for help.
Here are some online links, but if you google online AlAnon forum, or meetings you will find more I am sure
http://www.ola-is.org/olais/chat.htm
http://alcoholism.about.com/od/ola/AlAnon_Internet_Meetings.htm