To reiterate what others have already said, Step 4 work is not done with you. He does that work with his Sponsor. If you're truly done with this marriage, I would not meet with him to be honest. But that's me. He is trying to get you back in his life, inviting you to his graduation then wanting to include you in Step work. I think the boundary should start there. What will you gain from meeting him if he has another agenda? It will likely leave you upset again, at least in the moment. Maybe the kinder thing to do, if you want to set a boundary with him, is tell him he needs to do his recovery work with his Sponsor. Otherwise, I think he will continue to come up with reasons to contact you. That's not good for either of you.