My mom's iPhone 6 has been dropped into the toilet. Help me fix it!

Water immersion consequences along with how long a phone was immersed then disassembled to remove as much water as possible may or may not restore electronics since there are unseen areas where the smallest droplet may still short circuit unseen connections. No matter what's been said of using alcohol and other things to remove moisture, there are zero guarantees of success. Perhaps another way to address water immersion might be related to ac systems (vehicle, refrigerators, central hvac, freezers, etc); factory assembled then evacuated of air and moisture for a near perfect vacuum before introduction of refrigerant. A vacuum forces water or moisture to boil, evaporate faster at room temperature. If a partially disassembled phone were placed into a vacuum chamber with a vacuum lowering pressure, moisture can boil/evaporate readily without using alcohol to remove as much moisture clinging to hidden areas. Another way to accelerate moisture evaporation would be heat as in leaving a disassembled phone in the sun or low heated oven for an hour or more. Heat forces moisture evaporation. Moisture removal is only one issue. The other issue may be power.

The rechargeable battery may be damaged by water shorting its connections without creating a fire hazard. One way to check on battery power is removing it to measure its voltage. All lithium battery packs are labeled with voltage specs. Most have several pins and require careful probing with a multimeter, sometimes using a paper clip as probes to measure voltage. If a battery doesn't have voltage, either it died from damage or may take longer to recharge, an hour or more. Charging may require periodic voltage measurements to see if a dead battery comes back to life before the phone can start up. Does the charger output voltage? And can it be measured at the battery connections to ensure charging voltage is reaching the battery?

Power and electronics are two separate issues that have to be considered in phone restoration.