The first thing I would suggest is removing the idea that you have a right or wrong choice in this situation. If you feel like your friend needs to know what you saw then you should relay the information. Ultimately what happens isn't up to you in this situation. People decide to work things out or break them off for their own reasons and you don't need to feel responsible for that. I suggest not relaying the information with any bias and simply stating what you saw if you choose to go that route.
If you are feeling any sort of guilt for not saying anything, then it's probably your own way of telling yourself that keeping quiet isn't an option and repressing that emotion could lead to bigger issues in the future. I understand that you don't want to cause your friend pain by explaining what happened, but you aren't the one that did anything wrong here. If you are content with not saying anything, then you should be content with the fact that it could come out later and your friend may feel that you should have told them what you saw at the time to prevent the marriage. In the end, just do what you feel is right and you can't really have regrets about that as you have to live your own life in a way that you are comfortable doing.