(well said, Meg)
and in fact, my first thought when I finished reading your post was to step down from being in the wedding party but that sends a negative message to your friend, no matter how much you say it's not about your friendship but about one (darn) dress.
I too had tons of trouble judging what size I'd be when in my pgs and in my experience, 3 wks can have a lot of growth depending on where you are, in what fetal growth spurt.
See what another seamstress can do, see if something can be done at the back where the zipper is--? Or maybe some subtle panel additions to the sides?? A good seamstress will have clever ideas.
I certainly wouldn't pay 300$, is that how much the dress originally cost and you'd be buying a new dress??
If you can stand it for the 1-2hrs of the ceremony and slip into something more comfy for the rest, that works too, I'd say. All eyes will be on the bride anyways :D
good luck and be sure to keep us posted.