Your bridesmaid should have the dress tailored by the boutique’s in-house seamstress or go to one recommended by the designer, as they’ll be familiar with the designer’s dresses and know creative fixes to make the dress fit. For example, the seamstress can add a shawl across the back and shoulders if the dress can't be zipped up or cut a too-long hem to add panels to the side seams of the bodice.  Other adjustments like adding tulle straps to a strapless dress can help provide support for an expanding bust.