On HW alone I managed to find one news article and several old threads debunking the idea that activated charcoal can be used to treat IBD.
Here's the news article:
Beware claims that activated charcoal can cure gut troublesThe old threads can be found by doing a site search for activated charcoal ulcerative colitis (or crohn's).
Put simply, activated charcoal is just another in the long, long line of purported "cures" that turn out to be absolutely nothing of the kind.