My cousin in Ohio believed 5G towers were causing his headaches until I pulled up the coverage map
He was dead set on it for 3 weeks, then I showed him the map and how his headaches started before the towers even went up near his apartment. Now he switched to blaming his neighbor's wifi router. Where do you draw the line between a real concern and just looking for something to point at?
The thing people miss is that this stuff tends to run in families or friend groups, so once one person starts talking about towers or routers, everyone around them suddenly "feels" something too (kinda like how yawns spread, but with symptoms). Your cousin probably needs a hobby more than he needs a doctor, since a bored brain will grab onto any mystery it can find. Give him a project or a gym membership and watch how fast the headaches move on to the next thing.
I get what you're saying about it spreading between people, but I feel bad for your cousin honestly. My aunt went through something like this and everyone kept telling her it was all in her head or that she just needed a distraction, which made her feel crazy on top of feeling awful. Turns out she had real sinus problems that a doctor actually caught, so writing it off as boredom can backfire. A hobby doesn't hurt, but telling someone their pain is fake is a fast way to lose their trust.