I don't agre with handing him the letter in person... the material makes it pretty clear you should be avoiding him...
Re the exposure to hsi parents etc...
The book doenst' go into this as much as I would like, but you kinda have to divorce bust your exposure group too.. your in laws don't just need the facts of what he's doing, they need to see michele w davis' geurilla db video and hear how you want to save the marriage and what THEY can do about it.
I don't expect them or even want them to call their son names... i HOPE they will tell him OW will NEVER be welcome in our home or in this family... She is and always will be an itnerloper and a threat to your family... period
You want your exposure group educated, that's your job, to get THEM where YOU are so they can fight along wtih you.. just giving them the facts of your H's infidelity and NOT educating them on how to manage that is rolling dice...
But you sound like you are distancing yourself from the situation... that's definitely a good sign.