Do you still have one surviving bird from the first two you got? If so, this bird could possibly be a carrier of a disease that isn’t affecting it, but is killing the other birds. Typically this would be a virus. Did any of the birds that died have any marks on them? If so, this surviving bird could be killing them. I would take it to an avian vet for testing, and certainly not get any more birds until you can figure out what is going on.
In a situation where every bird dies, the most likely cause is a toxin. You need to go over everything you used to build the aviary and research the materials to find out if something might be toxic. This includes whatever you are using on the floor or ground. Has your friend inspected the aviary? An expert might be able to spot the problem. There are many things that are toxic to birds – both by ingestion or inhalation. Their respiratory tracts are very sensitive so anything with an odor can possibly be toxic. If this is outdoors, the temperature can be a factor. Again, you need to try to figure out what killed the birds before getting more.
Thank you for asking Lafeber,