I got a little lost as to what the actual question is because of the whole story, but here's what I think, and it may make it simple for you. First of all, the point of going to a wedding is to be supportive of the couple. If you don't support the wedding/vows (and it sounds like you didn't) you shouldn't have been there, even if you've know the girl for 14 years. By being there, you were saying, "I totally support this decision you are making and am happy for you both", which wasn't true. Friendship is a two way street, and she is obviously not putting as much into the friendship as you are. What about her makes her your friend? Just something to think about. Second, calling you 4 days in advance of her wedding wasn't really acting like your "best" friend. Ok, I think I found your question after reading it again. Were I you, I would invite your "friend" over for lunch and explain to her that you love her, and you are concerned about her being married to this guy. Yes, you will be jeopardizing your "friendship" with her. Tell her that because you love her so much, and friends are supposed to look out for each other, you wish her well, but can't in good heart support the vows, or throw her a shower. Yes, she will be pissed off at you, but probably only until she gets a divorce from the guy, and hopefully there won't be any children mixed up in that mess. She is very immature for a 27 year old "woman" and needs to grow up. When it all falls apart, and she matures, she will come crawling back realizing you were right. But if she doesn't, she wasn't really your friend at all. Just because you know someone for 14 years, does not make her your best friend. People grow apart even if they still speak on a regular basis. Sounds like you have grown up, and she hasn't. It sounds like you know the right thing to do, but the idea that she is your "best" friend is keeping you from doing it. You can't compromise your morals and values because of that. Have you ever noticed that the right thing to do is always the hardest. Throwing her a shower just to please her would be the easy thing to do. Standing up for your values is the hard and right thing to do. Simple as that. I bet your husband would be super proud of you for standing your moral ground with her. He is probably tired of the drama more than you are.
p.s. who cares about wedding etiquette! Compromising your morals and values takes precedence over that! Don't be silly, be a grown woman.