I picked dahlias for my wedding last September; I picked the morning of, but they lasted in plain old water in mason jars for *weeks* after. There were other flowers mixed in, too- some daisies, lots of interesting greenery, queen anne's lace- that lasted really well, too.

Is she looking at a PYO Flower Farm or the side of the road? A flower farm should be able to advise you based on what they have growing. The open fields and meadows, on the other hand, need more advanced planning.