iPhone 6s not turning on or charging even after hard reset
So, my girlfriend's iPhone 6s got water damaged from what she told me. She said water spilled on her phone while she was at work but she wiped it off fast and thought nothing serious so she continue using her phone like usual until she got back home from home. On that night her phone started acting weird and to the extend she couldn't use her phone at all and she tried to turn the phone off but it won't let her to do it. She got no choice but let the phone died itself overnight. Next morning she tried charging it but it won't charge. Nothing happens and can't be turn on anymore.
I took her phone and check if there is water damage on her phone by ejecting the sim tray and looking for the white indictor inside the sim tray slot. I saw the white mark and theres no any red mark so I wasn't sure if the water got inside the phone. i tried hard reset the phone and plugged it in to the computer but nothing works. I'm thinking to replace the parts inside of the phone but I'm not sure which parts to replace to make the iPhone alive again.
So, does anybody know how to fix a water damaged iPhone? or know which parts to replace? thank you so much for the time and really appreciate for the help!