How much filler is too much before it just cracks later?

I did basially the same thing on my old F100 bed sides a few summers back and I'd say he's mostly fine, with one caveat. One heavy pass over a quarter panel is actually how a lot of people do it, becuase filler cures by heat, not air, so burying it thick in spots is normal. What caused me trouble was the second big coat I slapped on a week later. It cracked within a year because I rushed the sanding and went over uncured material underneath. If he does his heavy coat, lets it sit a couple days in warm weather, then just uses the glazing putty for pinholes and shallow scratches like he's planning, that's the textbook way honestly. The real question is how thick he means by thick. If it's a quarter inch deep over damage that only needed an eighth, the extra material just sits there shrinking and flexing, and that's where you get those spider cracks along body lines a few years out. If it's a deep dent he didn't hammer out first, that's the lazy part worth calling out, not the single pass.

On the driver car outside thing, I'd say let him run it. My truck sits in the driveway year round and that cheap filler job still looks fine four years in. It's not show paint, nobody at the hardware store is inspecting the quarter panel with a straightedge. Worst case he's sanding it off in three summers and doing it again with better prep.

I'm on the other side of this one. I built up a rear fender on my Comet with two heavy passes about eight years ago and I've been paying for it ever since. The filler swells up around every rock chip and the edges have a raised ring that shows through paint no matter how much I block it. The bigger problem is that thick mud holds heat and moisture differently than the metal around it, and on a car parked outside in sun and rain the difference shows up fast. Thin coats over properly sanded metal, 36 grit scratches so it has somethig to bite, that's the way I'd push him. It really doesn't take much longer, the mixing is the same, you're just spreading it a couple times instead of once. A skim coat of glazing over the top is fine, but glazing is not structural and shouldn't be more than paper thin. As for driver quality, that's exactly the car I'd tell him to slow down on, because a garage queen might never see weather, but a daily driver outdoors will expose every shortcut within a couple summers. Sanding off a blob that cured wrong wastes way more of a weekend than laying it down right would have.

I've only ever done this on small stuff, like a rusty trailer fender and a dinged up motorcycle tank, so take this with a grain of salt. On the tank I got impatient and painted over filler that was still a bit soft underneath and it blistered within a season. Question for the group though, does he plan to get primer on it fairly quick? I always assumed bare filler left exposed outside soaks up moisture and goes bad faster, but I might be mixing that up with something I read somewhere.

The prep before the mud matters more than how it goes on. I did a trunk lid on my Galaxie and spent a whole Saturday with a hammer and dolly getting the metal within a sixteenth of level first, then needed barely two scoops of filler for the whole lid. Ten years later it still looks dead flat. If your neighbor is burying a dent instead of roughing it out, the filler is doing a job metal should do. Might be worth asking him if he owns a dolly at all.

The thing I'd ask him is how he plans to sand it. My first filler job I did by hand with a little pad and ended up with a wavy quarter that looked fine til the sun hit it. Next panel I used a long sanding board with some guide coat sprayed on and it came out flat as can be. Makes a bigger difference than most of the other stuff people stress over.

Update: I showed him the reply about hammering the metal close to level first and he actually grabbed a dolly from his dad's garage. Filler went on way thinner than that first blob, so that part's sorted. Still worried about jumping to paint too soon though. How long should he let it cure before sanding and priming? Cooler weather here lately.