What I get from you is :
- You wanna know why the battery drains when using GPS despite having
it plugged in:
Basically GPS is heavy on battery, explained here.Using GPS,as has been stated in the article, involves connecting to around 3 or so satellites at a very slow pace(50 bits per second) which means her phone at the time was using a lot of power.Depending on other factors in play namely
what other apps were running, what charger adapter she was using and
what cable was hooked to her phone from the car charger.
Let's start from the car adapter she was using. GS7 chargers are rated at 1.67A and 2.0A. Her car charger adapter most likely was rated lower meaning it wasn't producing enough power to sustain the power being utilized by the phone when functioning with GPS.
Note: Using a lower rated wouldn't explicitly mean your phone wouldn't charge it might charge extremely slowly or it might not charge at all or it might even discharge (this is when power output is less than 500ma)
Now if she happened to have been using a properly rated car charger, the cable she was using would most likely be the problem.Reference here.I don't want to go into that deeply open the link for more in-depth info but basically some knock-off cables don't take professional physics into plan and hence you get some 'wires' that were intended to charge cheaper phones not your Gs7 premium flagship phone. If you get low rated car charger + bad cable = no charging at all.Couple that with heavy power usage and you're basically doing more damage to your battery than helping it.
- Why her phone gets hot when charging(both during fast charging and
Well basically this is simple physics. P=IR^2 i.e Power=Current(I)*Resistance *Resistance(R^2) you're battery has internal resistance that's a constant. When using your charger with a fast charger and with the option enabled on your phone you'd notice it gets hot fast this is because fast charger inputs more current and your phone is trying to get as much of this current as it can.A basic principle world over is no machine is 100% efficient. The battery will take as much power as it can but will also have wasted energy.This is from the power that has been produced but isn't taken up by the battery. This wasted energy is emitted as heat.Which explains why the battery heats up both with fast chargers and normal chargers.
Try as much as you can to refrain from using accessories that weren't provided with your phone at purchase.Also refain from using fast charging all the time as a hotter battery = shorter lifespan refference
The last link also provides cons and pros of fast charging.