I would write something like this to her:
"Dear wife:
Your personal life is none of my business, but because I'm a kind person I'll take the time to send you a brief note. I understand it's much easy to blame the problems on others rather than sit with your husband and address the problems in your marriage, but let me tell you: there are many. In fact: they have been piling up for years. I'm afraid you guys are going to have to sit and have a honest conversation, (did i say "honest"?) . basically Your hubby is looking outside the marriage for something he's not getting at home: good sex and an emotional connection. I'm not sure about you but it hasn't been happening for him lately. However... if you decide to get all vindictive with him and serve him with divorce papers because of a couple lousy shags you'll be breaking your family, not me. I'm the syntom, not "the cause". Take responsability for your own problems, sit with him and ask him why he's hiring sex workers. You may not like the answer, but it will lead to a honest relationship.
best luck with all,
Then BLOCK.