This is reasonably straightforward - tell them now. It won't be a big deal. If you try to play with it in any way, it will be an issue.

I was going to the Caribbean with two of my friends who were getting married, and it turned out that I was on the same plane as them, me on the upper deck, and them in the back. I let them know in advance, and offered to take the wedding dress into the Club wardrobe if there was any difficulty with it. There were no issues. (The CW purser was nice enough to bring them down champagne at my request)

As Often1 says, you choose (or are able to) spend your resources differently. As you were not supposed to be flying together anyway, I can't see why there would be any difficulty accepting this.

C