It looks like the flowers are toast but they will be back next year. Worry about the plants themselves. From what I can see, they look pretty good, some burnt edges but overall, good.
Plants that small should not have been allowed to bloom in the first place. When a plant has flowers, they put all their energy and efforts into producing seed so the next generation will survive. These may be Hydrangeas that don't produce seeds but, in the plants mind, seeds are the all important end to this story.
Cut the flowers off and keep them well watered. Don't blame your neighbor, especially if you haven't spoken to him. Those plants were stressed from the beginning. Newly planted in hot weather and trying to support flowers. Way too much to ask of your new plants and your neighbor.