The LW got what she deserved. One can only hope she's bright enough to "get" the sarcasm. It sounds to me like there's more money than intelligence being applied to the situation.
Here's what Prudie's LW said: "I wanted to get to the bottom of this, as my mother-in-law had not even informed me that she had purchased anything. So, after the party, I sent her an e-mail, and she sent me a picture of the dress. I couldn't believe what I was seeing. My 51-year-old mother-in-law has picked out a dress with a wispy skirt, a V-neck, and spaghetti straps."
The choice of words is telling here. "...get to the bottom of this" ... would imply some evil conspiracy. Did she want her husband's mother to show up in one of those cute prairie numbers like the FLDS women?
You said: "....it's HER wedding. if someone very involved in the wedding, who was going to be in a lot of pictures, was going to wear something inappropriate i would definitely be a little upset and want to know what to do...it sounds like the MIL doesn't know the bride knows about her dress choice yet. the bride could call/email the MIL with a list of styles that would be "just beautiful" on the MIL."
Bad choice of words on your part there. Who is to say the dress is inappropriate? As a very smart friend of mine once said, there's no right way to do the wrong thing.
Under the circumstances, I am inclined to believe the bride's future MIL should be giving the bride advice on style and grace rather than the bride trying to dictate what her MIL should wear. A little sit-down to explain that there is going to be a wedding and not some farcical theatrical pageant might help Bridezilla get some perspective.