Life is a wild thing. An uncle of mine and his sweetheart split and raised families with other people. Fifteen or so years later, they were each divorced with children and found each other again by accident. They've been married to each other for nearly 10 years now, and seem reasonably happy together.
Sometimes things come back around. A lot of times they don't. Either way, we gotta keep moving.
Like I told a buddy of mine when he went through something very similar, "get a drink or something to eat, and ride the emotional coaster til it's over. Definitely don't say anything to her now. She's made her choices, and doesn't deserve to have a very positive landmark-event in her life tainted by the knowledge that her happiness is causing unhappiness in someone else".