I don't recommend outing him to his family. It will backfire in ways you wouldn't expect. They will believe what they want to believe, regardless of how much evidence is to the contrary.
Just think about the r'ship you want your D to have with these people and only do things that will not jeapordise the future of that relationship. You have control over that aspect, you don't have control over what they or h do RE their relationship with your d.
Hope that doesn't sound too harsh. Just a Purple outside looking in with perhaps a little more objectivity.
I would certainly not do anything to jeopardize D's relationship with her family. God knows I know what it's like to be cut off from family, and I would do anything to keep that from happening to her. And so I would never say anything in a mean or accusatory way in the letter I would send. However, I do need some closure with them--I have a lot of grief over this aspect of the divorce. Also, since he has put D in a position of keeping the secret of OW with her grandparents, this is a way of ending that issue. I hear what you're saying, and I will admit that I am not 100% committed to "outing" H, but when I do so it won't be so much an act of revenge or "outing" as it is a closure for our relationship.
M60 H52 D20 M14 yrs OW-old gf from 1986 bomb-5/18/08 H filed for D-9/10/08 D final 4/24/09 xH remarried (not OW) 2012
