I told him he needs a "cold spare", so if the first runs out of batteries, the second will still work, but he says he likes hot spares

Nonono, a cold spare is a flashlight sitting next to a fresh unopened package of batteries on a shelf in the closet.
Tell him the optimal redundancy strategy for a flashlight is a warm spare (ready to hand with the batteries in but not powered on).

I dislike the cold/warm distinctions. As the tradeoffs for warm usually make it cold or hot.

I think I agree if you use warm, my definition meets warm not cold

Also, if you can't put batteries on the flashlight, it's definitely not a spare when unbatteried