"but once breasts pass a C cup they're just a real challenge for cocktail dresses."

I have double D's and I own a cocktail dress, so I think she's confusing cocktail dresses and strapless dresses. There is an overlap, of course, but many cocktail dresses have straps and twenty seconds on google will show you that. Strapless bras don't work worth a damn once you're past a C cup, but put on a dress with straps and put a stitch through the bra strap and into the dress if it's so uber-formal you don't want to show anything.