I agree that the bride's weight, taste in dresses or cleavage preferences don't matter at all in this context. It is also irrelevant if the dress is considered bawdy, inappropriate or far too revealing by the salesperson, or if the salesperson would prefer a less modest, longer/shorter/offwhite dress on the bride.
It is any salesperson's job to find clothing the customer likes, and therefore, purchases. Even if the salesperson themselves would not be caught dead in it. The weight references sound very suspicious to me, too.
They are relevant to the fact that most larger women also have rather large busts which usually require some sort of consideration so that cleavage does not turn into a wardrobe malfunction.
The problem was that this was a wedding shop and they didn't have the bawdy costume the bride was looking for. Is the LW supposed to sell her something she doesn't have?
It seems to me that they wasted the LW's time and not the other way around.
They weren't looking for a dress for the bride to get married in, they were looking for something sexually revealing to keep the groom's attention
As far as I can see, the LW did nothing rude. She picked dresses and I would presume that given the fact that the bride and her entourage had the run of the place, they also picked dresses but couldn't find anything low cut enough.
Nowhere does the LW imply that she prevented the bride from trying on a low cut dress, just that she looked great in all the ones she tried but wasn't satisfied with any of them. She doesn't say that she tried to dissuade the bride from anything lower cut--the dresses on hand simply weren't low cut enough for this bride.
And no salesperson is obligated to refer you to somewhere else.
I don't think that any of this makes the bride a bridezilla--tacky maybe. The rude one was the person who blurted about the groom.