It's not necromantic - which is a thing in the setting but not within the scope of the Walkabout - and doesn't cause decay in the physical sense. To put in WoD terms because 5E isn't really my jam, it's a predatory Spirit/Prime entity that feeds on the quintessence inherent in a human's life pattern, not an Entropy construct. It's also, again, almost impossible to detect with magic. I cannot stress this enough.

I haven't been following his thread closely, but if he'd put together physical wasting and weird behaviour and jump to the conclusion of necromantic monster because that's what he's used to hunting/that's the most likely answer in his world, that's cool. You don't have to retcon a bunch of tags. Just keep in mind that it's factually incorrect in terms of how this setting works, and won't help him track the thing once it jumps hosts.