How should I handle this? Just hand her the letter? Anything to say when I give it to her? Maybe leave it by her horse's stall door in the stable?
Transfer the mail to her however you transfer other mail to her. And stop housing her horse. It's amazing that even the OM knows this is a key reason why your wife hasn't fully moved on.
It's amazing that even the OM knows this is a key reason why your wife hasn't fully moved on.