Obviously the plastic will be fine. The instructions should be the priority. Get them separated, open, flat and dried. Waxed paper should keep them from sticking if you choose to stack them to save on space. The decals will be a challenge to save. Get them fully dried then put each of them into a sandwich baggie and stuff them between the pages of a large heavy book. Later, when it is time to apply them, it will take much longer in warm soapy water to release from the backing, but patience will likely pay off. The boxes are probably mostly goners. Keeping them from warping may not likely be worth the effort. Save the box top artwork to at least keep it with the kit when it is reboxed. Keep circulating air around everything to help prevent mold growth. Hopefully his collection is a "I will build it" collection and not just an "Investment" collection because the water damage has killed each kit's value.