That sounds like fun!
I looked up a site I liked (linked below) and found a few possibilities for flowers that she might be able to find blooming at the right time.
Turtlehead is an elegant flower and would look great in a more compact bouquet or with other flowers or ivy trailing.
Queen Anns lace grows everywhere if she likes it and can keep it looking good after picking- try it.
I think milkweed flowers would look good. She may want to try to find different species that are starrier than the common one.
There is a native azalea that blooms in August. Maybe you can find some as that would be beautiful.
Meadow rue is very pretty and airy.
Meadowsweet?
Common white yarrow may look great en masse. Not sure about the smell for a wedding.
Many asters will be starting to bloom.
Boneset is white with small airy flowers (look out for bees) and blooms for a long time.

She can either use a mass of one flower or try to combine them into something more elaborate. She's probably pretty busy, though. It might depend on what you can locate and test out for durability before the wedding. Good luck!

Here is a link that might be useful: White Wildflowers