um... yeah. You really needed to do a necropsy with tissue work on that boa to start making claims of IBD. There are multiple things that can cause the neuro issues, now you may never know what caused it and it's probably too late for IBD testing as you froze the body (never freeze, always refrigerate if you want to have a necropsy done!). It could have been a toxin, overheating, parasites/virus, IBD, possibly other things I can't think of right now. Also the thing with 'a year' is more for pythons I believe. IBD can stay dormant in boids for a long time from what I've read.
I went through an IBD scare (wrote it up on this forum a couple years ago) - turned out after necropsy and her tissues were sent for pathology testing (brain, liver, I forget what else, I think 5 samples were taken and sent to UC Davis) - it wasn't IBD. She had parasites that got to her brain. It still sucked (and was quite expensive) but having the peace of mind knowing my collection did not have IBD was so worth it. Now you don't know what is going on. :/
Keep a close eye on everything you have, if it was IBD, it's not known exactly how it's transmitted yet (theories are mites and simply air transmission). I hope it works out for you...
I'm sorry for you and your friend's loss
edit to add - yeah beheading is never a good way to put down snakes, their brains can stay active for up to an hour after beheading, they need to either be put down at the vet (mine was put down via heart stick) or blunt force head trama to destroy the brain (gunshot works I guess) but again, then you destroy the brain which is one of the tissues that needs to be sent to lab for testing