There are a lot of things you can say. I start with my choice from the big 5:
"Oh"
"Wow"
"Really"
"Huh"
"You Don't Say"
and #6 from my dear friend Hangin In:
"How 'bout that?"
And of course there's always , "You may be right" and my personal favorite "It will be interesting to see how that works out for you".
It's difficult when one person in the relationship chooses a path of recovery. Behaviors change, reactions change, the steps in the dance change. There's bound to be some stumbling as you figure out new ways of dealing with things.
Recovery is a process, and your situation can improve whether the A chooses recovery or not!