While it is technically not illegal to feed coyotes, it is not a wise action. What your neighbor may not realize is that by feeding coyotes they are making these wild animals less wary of humans. That can lead to all kinds of problems such as the coyotes getting into trash or pet food that is left outside. It also has the potential to lead to predator/pet interactions (i.e. neighbor's small pets may be attacked and/ or killed). Once the coyotes start damaging property or killing pets, a nuisance wildlife control operator must usually be called in to remove the animal. While your neighbor may think they are helping the wildlife, in reality supplemental feeding usually just ends up causing the animals problems in the long run. Coyotes and other wildlife are very capable of providing themselves with sufficient food, even during the winter.
I don't know if the neighbor is attracting deer as well. If they are, your neighbor should also be made aware that it is illegal to develop feeding stations for deer. This law was made to help avoid the congregating of deer and facilitating the spread of the Chronic Wasting Disease.
If the coyotes are causing property damage, you can call your local Illinois Department of Natural Resources district wildlife biologist for advice on how to handle the situation. Sometimes hazing the coyotes with noises, whistles, etc. (depending upon City ordinances) can be helpful in keeping the coyotes off of your property. The DWB contact info. can be found on the Find a Wildlife Professional page of this website.
The Coyote page on the Wildlife Directory page can provide more information. It can be accessed at: http://web.extension.uiuc.edu/wildlife/directory_show.cfm?species=coyote
If you would like, I can send you some brochures about this website that you could give to your neighbors to help educate them about these issues. You would just need to to e-mail me your mailing address.