When they did the ERCP they should have visualized the common bile duct and the pancreatic duct using a specific dye. You need to find out if they were able to do that and what they found at that point. If the pancreatic duct was blocked, a biopsy of the cellular lining of the duct should also have been done expecially if the blockage was in the beginning of the duct that leads into the pancreas.
Talk to the doc about what they found when they did the ERCP. You want to see the written results of the test. It will give you more information that what you're currently getting from the doc.
I think your mother (or someone) need to clear this out with the doctor(s) who said that pancreas was enlarged (1. on basis of what investigation this was said?) and then, that it is not (2. with what investigation this was confirmed?) 3. Was the removed stone blocking the pancreatic duct (this would explain pancreatic enlargment, which vanished after the stone removal)?
I've listed some questions for the doc. If any doubts remain, the ultrasound of pancreas may show, if there is any mass or not. Endoscopic ultrasound is even more accurate.