That is a nice gesture for the woman to want to write the letter BUT i would not let her write it. She might have good intentions but your wife might not see it that way. Matter of fact, if I was your wife, I would assume that you were the one that put this "lady" up to writing a letter to me. I don't know, a letter would put me off and make me think that you were weak as a man to have another woman address me about our issues.
btw, i know that you are not that way just saying if i was in your wife's shoes. I don't want my husband sending another lady to tell me something about our relationship.
Me: 28 H: 32 1st marriage 4 both 1 1/2 year married 2gether for 9 1S: 6months 1stepson: 2yo