He was wearing chest-waders. The big problem with chest-waders is that, if the harsh felt soles do not grip the slimy rocks and you fall in, the water goes half way down to your legs, then stops in an air-lock.
The air-lock forms a couple of balloons on both feet, and your feet rise to the surface of the river, leaving your head below the water. The pressure of the water (along with the other pressures) means that you have no way of releasing the waders, and you simply drown. This is what happened to my friend's friend.
There is a little vest you can buy, which automatically inflates as soon as you hit the water. It costs about £150, and is designed for fishermen. A small price to pay, under these circumstances. All you have to do is make sure it is worn in conjunction with short waders, so the air at the top is not competing with the air at the bottom.
If this public service announcement saves but one life, then that is one more salmon which will end up on the plate of the family at home, waiting for the angler's return.