So she doesn't like the dress but she didn't think it was a big enough deal to tell you, she didn't want to hurt your feelings. And then your coworkers decided to hurt your feelings by telling you her thoughts on the dress? Who tells people the mean things they overhear about them? Your friend/bridesmaids shouldn't be venting about the dress, that's rude. It's also rude of your coworkers to run to you and tell. Sounds like people getting off on drama. This isn't a real issue, it's the people around you. I would let it all go, the more you feed into this the worse it will get.