Room-mate broke lease, left belongings, is now asking for his security deposit back.?

I personally think it might be best to go to court or at least seek legal advice because your situation with this roommate is so complicated. I've attached a link which is Michigan's practical guide to tenant/landlord rights. It gives you advice about what to do in situations like these. With regards to the security deposit, you have to check your lease about breaking it and refunding any person's security deposit, and I would go talk to management myself or call them to ask their advice on the situation as well.