I would send a brief private message to her first, being upfront about who you are and why you are writing. Don't be too cryptic, as she might ignore you. (Lots of random weirdo guys send messages to women on facebook). On the other hand, wait until she responds to you to give her the details about your evidence. You could just send the first 5 paragraphs of your draft and add that you have evidence if she would like to know more. Do give her your phone number.
It wouldn't hurt to try to reach her on facebook. There is an option to be immediately notified of FB messages in your regular email, so if she has set it up that way, she may get your message even if she doesn't check FB often.
I don't know about telling her "Ask him about this". Hopefully the exposure pros will weigh in here, but seems to me that rather than "ask" about it, once you have the evidence, to say "I know all about your affair"
