my thoughts exactly it's quick alsoIn my area, July is when Mosquito sprays get started. It's quite possible that is the problem for your friend as well. Someone in the close area is spraying for Mosquitoes or some other pesticide spray for something. It could also be close neighbor spraying in the garden or around the house from a pesticide company. Anyway, I'm like most others here, someone close by is spraying something.
Twice I have had bees accidentally poisoned. Once it was a gardener dusting their plants with sevin (including the blooms). I came home one day to a shovel full of white powdery bees laying in front of the hive. Many make it back to the hive then die when they are not foraging very far. Both times were in my old neighborhood where the lots are about .25 to .3 of an acre. The second time, I looked over and saw a neighbor 2 yards away spraying their apple trees to get rid of the large spider webs in the tree. It was loaded with blooms and my bees were all over them. I could literally see the stream of bees going to and from hives and apple trees. I ran over there and instructed them to never spray blooms with insecticides. Also said if you feel you must then do it late in the evening and let me know ahead of time.Look for some one growing vegetables close by and spraying sevin on his stuff. I could not keep colonies alive in one area and chanced upon a guy spraying plants close by, asked him what he was spraying, Sevin was the reply. I removed everything from that little yard and never went back.
a good optionOnly solution I could offer is move the hives. Much as he may like them at his house, is the pain of seeing this happen every year worth it?