Who am I thinking of? I only know Sasha Siemel through Peter Capstick's book and I remember him discussing a jaguar spear hunter who was killed in one of these encounters (which is hardly surprising).
I think you’re remembering the Siemel chapter opening describing the Matto Grosso native that was hunted by the man eating Jaguar, which is just about all of them. That set the scene.
Siemel’s spear hunting teacher might have died in a hunt,but my memory is not clear on that.