Do I ignore it, because I can't or do I hope that the person and their finance split up?
You ignore it and move on. There are billions of people on the planet, millions in your own country and I dare say thousands within walking distance of you. You need to realise that you are not living life in a movie, where the bride will drop everything and run off with the man who has a crush on her, it's not going to happen.
Woooahhh did you assume just two genders?We need more context.
Is it a she or he?
Is she an ex?
Did you both have a spark but never acted on it?
Were you her stalker?
Is she local, or someone on TV?
Were you not bothered when they were just courting?
If you were then why didn't you do something then?