If I was the one affected, yes if I thought that it would make me feel better. As the parent, I wouldn't try and influence them either way. My oldest child had a terrible first experience with his first-second grade teacher, she admitted to me in a parent-teacher interview once that she hated being in the same room as him! I felt like I'd been punched in the guts but not once did I ever put her down in front of my son. Eventually we removed him and changed schools. I felt kind of sorry for the school in the end because it proved that they had failed as a whole to provide the necessary strategies to help him succeed. The school he is at now bends over backwards to find what works for them and him. (Turns out there's a good chance he has Aspergers).
I get the impression your son is very level headed and I'm sure he won't be sorry for writing to her if he remains civil about the whole thing. It sounds like he already has an idea of what he wants to say and nothing about it seems wrong to me. I would only wonder what his objective is. To make him feel better or to make her feel worse? I guess the question is do you have any idea of how to contact her or do you need to become a private investigator to find out? May not be worth all the hassle.