Donations at weddings are extremely frowned upon on TK, just to let you know. Read the sticky at the top of the board.
We're choosing to let the guest know b/c #1 It's our way of honoring his mother and acknowledging her sacrifice to the disease. And #2, it's a small wedding, and everyone there will see the sentimental value in it and hopefully one day make a contribution on their own so that the donation is no longer just for her but also to kick cancer's a**. It's not for everyone, but I know that our donation on behalf of our guest will be viewed by them in a positive manner, and being as how they still feel the loss of his mother, will appreciate it much more then some bubbles or a cookie.
Though many people on TK don't like donations, I think they are a great idea and your wording was perfect. As a guest, I don't feel like I am entitled to a favor so why would I be mad that you donated money instead of giving me a meaningless gift? I would love it. I'd rather see that your extra money went toward saving a life than a chocolate bar or candy.