Those spiders absolutely thrive where I'm at, no telling how many times I've been bit but I know of two confirmed. I've studied these bastards, I know how they think, how they survive, where they hide, I hate every single one of them. Anyway, what prompted the emergency for EMS and then the ER? Was he having a panic attack or something unrelated, maybe a strange allergic reaction? How many days between the bite and amputation? I have two theories what happened, 1, Most of your body has thicker layers of skin, fat, and muscle, those layers provide an immune defense and barriers to deplete the toxins, maybe since it was on the thin layered hand, the toxins could quickly spread into tendons, ligaments and maybe even bone? 2. Several days into care, the open wound got infected, probably staph, early infection warning signs ignored.
The effects of a Brown Recluse bite is usually nothing to worry about, I bet most people never know they've been bitten. A low percentage of bites do produce those nasty necrotic wounds. Some of these wounds are probably better called an "Ulcer". Deep wounds (Ulcer) are very susceptible to infections and that's where most people have their problems, sadly they probably infected their own wound using poor wound care practices.