Why would you send the letter, regardless of how short? Bringing family into this isn't a wise decision. Again, if you enjoy his company, meet him for lunch, talk about fishing or whatever. He already knows how you feel about his D. He's going to support her regardless of what she does, or how he feels about you. So, really, other than opening a second front to confront your W about her decisions (which sounds like your back of mind intent here) that will just tick off your W and cause her to dig in her heels even further, why sent him a letter?
Don't screw yourself over.
Your W seems to be interested. Don't push her or chase after her. Also, prepare for her to run away again. Did you ever see the castle analogy post?
My personal feelings is that it's OK to tell your W is she looks nice, just don't leer or anything.
BD
My latest
Me: 36 W: 35 2 D: 9 and 5 T: 16 years M: 12 10/4/06: Bomb 10/5/06: Ended A 4/22/07: ILYBNILWY