I'm no expert. Obviously. But I wouldn't write the letter. It might sound like an ultimatum and she might not respond the way you want. Is a last resort letter one of DB or DR techniques? I haven't seen it in DR but I haven't read DB. So if it's not one of the techniques or if it isn't part of your coaching tips, I wouldn't do it. But don't just take my word for it, wait for one of the experts to respond before you give her any letter.
Or you can write the letter as some sort of therapy exercise and then destroy it. You know, for your eyes only. That way you can get it off of your chest but not cause any damage.
Me:28 H:30 D:3 M:6.5 T:7.5 BD: 10-27-13 H moved out: 11-01-13 Handling other paperwork before petition is to be filed.