iPhone 6 does not power on or charge

I recently had a customer bring an iphone 6 in along with her other iPhone 6. We used the screen from the 'broken' one to install on the working one because her screen was cracked.

She left me with the phone. It does not power on, nor does it take or hold a charge. It's completely dead. She didn't give me a history on the phone at all. She said it was her moms old phone and that it didn't work at all.

I don't see signs of water damage within the phone. I'm wondering what steps you guys would recommend taking to diagnose what is wrong with the phone.

My thoughts were battery and charging port. Am I on the right track? Would you suggest ordering anything else up front to find the problem?

I'm relatively new to the repair business