Your wife has mentioned her ex to you several times -- telling you that although he was not always a great boyfriend, the sex with him was always incredible -- much better, she said, than it ever has been with you. As painful as it has been to hear that from her, you have consoled yourself on those occasions by reminding yourself that it is YOU that she is married to, not him -- and you assumed she would never see him or be with him again. You were wrong.
And as for this "coincidence" -- you are pretty sure that she had made contact with him again over facebook and had told him where you and she were going in order to "celebrate" your fifth anniversary. So it seems now that she'll be having a lot of great "fifth anniversary" sex -- but it won't be with you.