IBD is determined through blood testing and pathology testing on various internal tissues gathered through necropsy.
I know this because I had an IBD scare years ago and euthanized two of my boas for full testing (luckily they were both negative). It was very expensive and results took weeks to get back.
What exactly did your vet do and how did they determine that is what it was? Something doesn't add up.
I'm sorry your little snake died.