I understand the panels have some reinforcement but it's not designed to stand up to the type of pressure we put it under. My rear quarter panel has a big metal bar going down the middle of it but it was still flexing like crazy. Slapped a couple CLD tiles on it and you wouldn't believe the difference. I know sound deadener is not the end all be all for noise issues but for flexing it's definitely one of the most effective options.
I do agree that he should check for loose bolts first, but just because he's got flex doesn't mean something has failed, he's putting the car into conditions it was never designed for.