I believe the main reason for these incessant vocalizations is the fact that she has 4 or 5 roosters (she doesn't even know) and only 5-6 hens. My research on this site tells me the correct ratio should be 1 rooster per ten hens.
The ratio of 1 rooster to 10 hens is not relevant here.
That ratio is correct for commercial breeders with hundreds and thousands of hens & roosters. Roosters eat feed but do not lay eggs, so those breeders want to keep the fewest possible roosters while still having all the eggs be fertile. 1 rooster for 10 hens is about right for some breeds, while others need more roosters (as high as 1 rooster per 5 or 6 hens) and others can get by with less (1 rooster for 12 to 15 hens.) But since your neighbor is not trying for maximum fertile eggs per rooster fed, the commercial ratio does not matter.
People breeding chickens on a smaller scale (like for shows) will sometimes keep equal numbers of roosters and hens, or a ratio of one rooster to two hens, or various other numbers. It depends on the breed, the individual chickens, and the owner's goals.
For people keeping chickens as pets, who do not care if the eggs are fertile or not, ANY ratio can work as long as the individual chickens are not causing issues for each other (like roosters injuring each other in fights, or roosters injuring hens by over-mating.)
Some people even keep flocks of just roosters (sometimes called a "bachelor flock.") That can work fine too.
Some people raise roosters for specific purposes (like feathers for flytying and other crafts), but of course raising roosters for that has nothing to do with any rooster/hen ratio.
None of those has anything to do with the amount of noise the roosters will or will not make, just like none of them has anything to do with what chickens would "naturally" do in the wild, or what would make any specific chicken "happy."
As regards your neighbor's chickens, if she kept the males & females in separate pens, that would guarantee no overmating of the hens, but would probably not do anything to fix the noise.
Personally, I do not think that more hens would make your neighbor's roosters any quieter, but of course I could be wrong about that.
I'm going before the board of supervisors to encourage them to amend the ordinances so they are empowered to address situations like mine by adding specific language about keeping an acceptable rooster to hen ratio, adding in language to address nuisance animals, and also something that addresses soundproofing. I don't expect much to happen but it will be a start at least.
No matter what rooster to hen ratio you choose, someone will be able to provide good evidence of times when it is wrong for one purpose or another. So I suggest you make sure you have good points to support whatever number you suggest.
I suspect it would work better to have a limit on roosters-per-acre, or some kind of total noise limit, rather than trying for a rule about rooster/hen ratios.