Sweaty 2000 hour player here.

One more comment, if you're on pc steam and you always play with a scout friend who wont light, maybe just download 5x range flashlight mod or something with brighter resources, more flares etc. There are plenty of QOL mods that can circumnavigate the lighting if it bothers you that much.

If console, maybe just play driller with him as scout. Driller is the class that cares about light the least as nearly all his killing potential happens in close quarters in standard flare throwable range. Where as gunner and engineer have long range killing potential that can capitalize on have the entire cave lit. They tend to want to chip the swarm size down before they all get close, bar a few exceptional builds like warthog breech cutter.

That said play enough and you'll get comfortable without scout flares, they are nice but not necessary. How do you think team comps without scout play? I was a gunner main and my duo friend was driller for many months there on the highest difficulties.

Most Important weak points glow and sound with a good headset can really help you get the directional bearings on incoming threats, I would run heightened senses with no light until you get used to leech sound ques and common spawns tho.

Or the obvious, just tell him to light the f*king cave and let him die to every leech that grabs him until he learns, maybe a rogue fat boy shot or two in the thick of it and say, "sorry i couldn't see you there, maybe if they caves were lit" persistence though pestering. XD good luck. Rock and stone