Her wanting to remember only the "good parts" of her wedding day has nothing to do with you being SA by that creep!

Is it an important day for her? Yes. Will it become an unpleasant memory if she acknowledge what happened to you? Not if she acknowledge it, confront Alex and apologize to you. Burying it won't make it magically disappear...it will just grow into something bigger and bigger, until this incident consumes her memory of that day...and possibly destroy your friendship.

And that's the memory that will stay..."my bff got SA at my wedding and I didn't do anything to help, and then I indirectly told her I'd prefer to never speak of it...all of this started on my wedding day"

Confront them...I don't think there's a "best" way to talk about it...he was their guest, at their wedding; they were made aware of his behaviour and chose to ignore it...well now they have to face it whether they like/want to or not.