What Causes Undigested Pills In Stools?

My Dad 66 has been passing all of his pills undigested for past few weeks if not longer. I've done a sample test on a couple to try and dissolve the Metformin and after 2 days has not dissolved in soda. He also has had loose stools since gall bladder was removed nearly 22 year ago. But this is first time we have noticed pills in the toilet not dissolved. He's going for colonoscopy tomorrow. What would cause the pills not to dissolve? He is getting an ct s can in the coming weeks and is being seen by a gastroenterologist. And has had a ultra sound with no results. Thanks P.Harvey