I found that I can be supportive without trying to fix or enable as long as I toss the ball back into the other person's court. I do that by saying positive or encouraging things like-"I know you can do this" "Do what you feel is best" or any other thing that places the responsibilty for action on the other person....instead of ME.
I have an inlaw much like yours and when it comes to dealing with him...I respond when he crosses my boundaries and only then. If my inlaw crosses somebody else in my family...then that person needs to address it on their own. It's very difficult but the inlaw has learned to think twice before he does his nasty 'stuff' tearing others down.
There were many times when I offered my opinions when they were not welcome. For me to mention something like a type of rehab or recovery program without being asked is not something I will do twice. Maybe once, but any more after that is me trying to influence an outcome that I want.
When I took my grimy little hands off of my son's business...you know what? (plse see my sig line) Things got worse in many ways...I stopped doing what I was doing...and upset the status quo. I got better and he got worse before he got better.
At least now I know that his recovery belongs to him and not me. btw...He is doing great,has been clean for two years.
I attend Alanon because I need the fellowship of others like myself. In case you haven't tried a meeting yet- I highly recommend it.