As far as I know, there's nothing wrong with coffee or caffeinated drinks (as far as bm's go). I think caffeine can actually act as a laxative or promote looseness. The reason I suggested non-caffeinated is because it has also been said to act as a diuretic, although I think that's debated. In any case, if it's true, then drinking non-caffeinated beverages would allow their body to retain more water, and if someone's bulking up on the fiber, it's also a good idea to make sure they take in enough water to loosen up all of that bulk, otherwise they might end up feeling uncomfortable.
If you just eat food that is high in fiber you should poop more. Basically anything that is not an animal will make you poop. Caffine is fine and it might make you poop quicker since it will get those poopin muscles moving more. Water is good to otherwise you might have hurty poops.