I guess I should write an update - I almost bought a new flashlight but still decided I'd take a closer look at the Fenix. I have no idea what Dad was doing with it but I have actually managed to get it working properly again. I simply disassembled the light, cleaned everything, put in fresh batteries and ZAP, sure enough there was the bright, white 180 lumen beam again.

I will have to do some more testing but so far it's looking good. Don't ask me what happened. I guess the LED is still intact, though.