Friends phone turned off mid use, they plugged it in but it has no charging light, tried a hard reboot and connecting to...

honestly i would try changing the battery before anything it sounds like it is not getting enough juice . i would also contact apple and see if they can run a diagnostics on it.

It looks like Ifixit has a variety of Nintendo Switch Parts for sale and even repair guides!

I would check the power cable and make sure it isnt getting hot. I would then make sure the power supply fan is turning that way you know its on and not going to overheat. that should diagnose the power supply and the temperature. anything farther will probably require opening it up.