Use this teardown to start your water damage repair. It's not a definite tear down, but it brings you to taking out the motherboard at least.
Then use the Water Damage Guide for how to clean up the phone. Be very careful. The guide really is important because you don't want further corrosion in your phone.
If it doesn't power up still, I would look into another battery. As long as the phone doesn't have a password or code and the device still turns on, you should be able to connect it to a PC to get your customer's pictures.
Please keep us updated and good luck!
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