Defamation/slander

Need to know a lot more about the contents of the letters and the context in which they were written to say anything useful. Defamation is a complex and technical legal area, and there are tantalising clues in your post whcih suggest that, even if the letters are defamatory, various defences might or might not be open to the author of the letters. You mention "adjudication proceedings"; privilege (which might be a defence to any defamation action) may attach to a letter written in the conext of proceedings. Or there are a couple of other defences that could be relevant.

You're not going to get anything useful here unless you are in a postion to be much more open, and give much more detail, than the OP. And I understand why you might be slow to do that. I think the best advice you can get is for your OH to take themself and the letters off to a solicitor to whom they can speak frankly, and in confidence.