I am wondering if all this really comes down to the LW needing to know more about her stock of dresses and how they will fit various body types.
I am speaking from the standpoint of someone who has more than the average amount to put in the top of a dress, just for the record. And I will say, it's a little challenging sometimes to find clothes, and the proper underpinnings to put under them, that make me look "sexy" but don't leave me constantly worrying about a "wardrobe malfunction".
From my experience with both myself and with friends and family shopping for dresses and other clothes, it's a lot easier for a thin person, or a person who is not disproportionately larger on top, to find lower cut shirts that can be more comfortably worn without hovering on the brink of disaster. No matter how sexy you want to look, even if you want to look "sleazy" or "sl*tty", you really don't want to suddenly be totally exposed, KWIM? You want to feel in control of your own clothing.
So maybe the LW did not understand her merchandise well enough, and was afraid to bring out anything as low cut as what the BTB seemed to want, because she didn't know which of the lowest-cut dresses could be worn with the least risk of wardrobe malfunction by a larger person (just up top or all over) and what underpinnings to advise the BTB to wear to make the dress fit properly and show maximum cleavage without letting the dress fall off or reveal more than intended.
And that's not even taking into account the fact that many bridal stores don't actually carry gowns in plus sizes. You have to hold up an unzipped size 8 (or whatever) while the saleslady holds the dress up in the back and try your best to get an idea of how you'll actually look if you buy the dress in your size. If you are not small and you are going for maximum cleavage, "trying on" dresses this way, especially without the proper underpinnings, could be a real challenge-a challenge that maybe the LW was not up for.
So, bottom line to me is that maybe the LW was not "rude" in the sense of it being deliberate, but she could have done a better job with this BTB had she had more knowledge of her stock and more confidence to try to help someone in a non-average size find a dress. IMHO of course.
And BTW, that's a great point somebody had about how maybe the GTB/BM scrabble occurred before the GTB and BTB got together. But it doesn't make the MOG's comment any less rude. I would have been so embarrassed if I were the BTB.