I'm not sure why DAN would have so informed as the following from their Medical FAQS has this to say about the matter:
"Diving Medicine FAQs
Crohns Disease and Diving
Q: I have a new student with a history of Crohn's disease who has had a colostomy for 20 years. This student is in excellent health, has had no further surgery and is on no medications. Would having this in any way restrict this individual from participating in scuba diving?
A: Crohn's disease is a chronic inflammatory disorder of the digestive tract, which can lead to bowel obstruction, perforation, bleeding, and other complications. Surgery is sometimes necessary to remove the affected area of bowel and a colostomy (surgical formation of an artificial colon opening) is sometimes performed.
Individuals who are free of any symptoms of Crohn's disease should be able to dive without difficulty. An individual with an ostomy (an artificial opening) is at no greater risk for injury while scuba diving than any other diver. However, the Koch pouch, which is airtight and requires a catheter to relieve pressure, is a contraindication to diving."
And even that doesn't seem to be entirely accurate. Because a properly functioning Koch pouch is airtight, if one could insure that there was no air in the system, then Boyle's law would have no implications for a sac filled entirely with liquids or solids.
Perhaps the most prudent course of action would be to let the diver's gastroenterologist make the decision, perhaps after speaking with an actual DAN physician knowledgeable in pouching systems.
This is educational only and does not constitute or imply a doctor-patient relationship. It is not medical advice to you or any other individual, and should not be construed as such.