[candy_coated_bitch]

In my opinion, you shouldn't force yourself to forgive someone or accept their apology if you don't. I've had falling outs with certain people in my past that I don't think I'd ever really want to forgive, per se. So I get it. You have a right to your feelings. Forgiveness is kind of a tricky subject but I'm pretty sure it's not something you can FORCE yourself to do. It doesn't sound like you want that.

I would say if you feel you really must acknowledge the email do so in a short, genuine way that feels comfortable to you. You can acknowledge that they are doing important work for themselves without having to necessarily be accepting of what they are telling you. I would probably say something along the lines of being glad they are in the program and taking important steps to improve their life and wish them luck. Why even address the incident that happened at all?