Of course, they cannot guarantee it, because it would give someone the ability to destroy their iPhone with water and claim it under warranty. But if you’ve seen water tests, etc on YouTube it is obvious that (assuming nothing has been modified) it will survive just fine. I’m telling this person not to worry, because they shouldn’t. While it is against Apple’s advice, people record underwater with their iPhone and take it in the shower, wash it sometimes, etc and it is fine. The rating given by Apple has even been proven to be an underestimation of its capabilities. So, in conclusion: don’t worry, but don’t do anything extreme as Apple doesn’t cover any water damage.