I doubt it is the sunflower seeds as well. My peafowl's main food that I give them is sunflower seeds. I give them other food as well such as cat food, vegetables, etc, but the food they get the most of is sunflower seeds.
I would definitely lean to it being a broken neck, but as you all have mentioned we are not sure if the bird was showing signs of being sick, and the owner just didn't know what the signs of a sick bird look like. Here is an article I wrote a bit ago about identifying when a peafowl is sick:
http://bamboopeacock.com/Knowing_When_a_Peafowl_is_Sick.html
If you have known your birds for a while, it is very easy to notice when they are not feeling well, but if you haven't had them for long it can be hard to know what is normal behavior and what is unhealthy behavior.
Regardless of what the issue is, if these birds are in a dog kennel, the top of it
should NOT be chain link fencing! I have had issues with raccoons before so I wondered if I should be using fencing on the top instead of netting. I remembered reading about netting being safer for the birds, but it was time for me to do my own research into the subject. What I found was very sad. Many people have lost birds because they used chain link fencing (or some kind of fencing) for the top of their pen. The peafowl spooked, flew up, and broke its neck.
My first peafowl pen was a dog kennel. It wasn't a very nice pen for 2 adult peafowl- but it was temporary until I turned them lose. We covered the top of the dog kennel with a plastic tarp. I would recommend using a tarp instead of fencing over the top, just make sure you replace the tarp whenever it starts to become brittle and make the peafowl a larger pen as soon as possible. A chain link top is also dangerous for new peafowl. New peafowl are almost always flighty because they are not used to you and will fly up and hit the top often until they calm down and get used to you.
I think chicken wire is safer than other types of fencing for a top. I know several peafowl breeders that use chicken wire, but often it is smart to look to large peafowl breeders to see what they do, since they have some of the most experience. Most if not all of the pens large scale peafowl breeders have are covered in netting. The netting that is probably the best is toprite 2 in. knotted netting.
We are all just guessing what the cause of death was, but if it really was so sudden with no signs of illness at all, that really sounds like the peacock flew up and broke his neck. A dog could have spooked him, etc. Anything can spook them that is why chain link is usually a no no.