Don't let her lay a guilt trip on you about medicated feed. There are a whole lot of people that don't use medicated feed and don't have problems. Your problem is not due to medicated feed or not-medicated feed.
In the dosage in medicated feed, Amprolium does not treat a dangerous case of Cocci. It does not even prevent Cocci, they can still come down with it even if they eat medicated feed. What it doe is interfere with that bug reproducing. It does not totally stop reproduction, just slows the reproduction down.
Just having the Cocci bug in their system is not a bad thing. In some ways it is good because in two or three weeks the chicken will have developed an immunity. The problem comes when the number of the cocci bugs gets out of control.
A normal sequence is that the chicken eats a few oocysts (consider these Cocci bug eggs). Those oocysts "hatch" inside the chicken and burrow into the intestine walls. After a few days they start developing oocysts that are expelled out the chickens rear end. If these oocysts stay wet in an area that has chicken poop in it, like a dirty waterer or a wet brooder, coop, or run for about two days they develop to a point that they can hatch in a chickens intestines if they are eaten. If the water is clean or the brooder, coop, or run is dry the oocysts don't develop. Coccidiosis is highly unlikely to develop. If the water is dirty or conditions are wet, they can eat enough developing oocysts for the numbers to get out of hand. But this takes time. It's usually at least three weeks after the first infestation before you have a problem, and often longer.
Your timeline does not sound right. If the problem were Cocci and the medicated feed was what was keeping it in check even if they had filthy water or a wet poopy brooder, it should take longer than a day or two for those numbers to grow to dangerous levels. But if the conditions are that filthy the numbers can grow dangerous even with medicated feed. But you said the water and brooder were clean. I'd guess your conditions were not filthy either.
There are different strains of the Cocci bug. Some are worse than others, mainly because different strains attack different sections of their digestive system. That's why some strains cause bleeding but most don't. Some can be more dangerous than others.
I don't use medicated feed. I keep the water clean and the brooder dry and I feed them dirt from my run, starting on the second day they are in the brooder. By the time they get out of the brooder and hit the ground, they have developed the immunity they need for the strain of Cocci I have in my dirt. Your broody hens do the same type of thing, making sure they get introduced to that bug when they are very young.
I don't know what was causing that pasty butt. It sounds like that probably had a big part in your problem. But pasty butt usually doesn't affect chicks over a few days old. So again that's confusing. To me it sounds like they brought the problem with them, it was nothing you or your friend did.
I can't solve your problem, but switching from medicated to unmedicated was not the cause.
