Water in the amount sufficient for an all day hike is not really "tiny" or featherweight (1/2 gal minimum). So the backpack must be comfortable and the person must know how to wear it comfortably. Camelback type with the waist strap and chest strap engaged is the way to go. Almost unnoticeable, especially if you allow enough space between it and your back on a hot day. So, I would start from explaining obvious benefits of hydration, point to the most unobtrusive way to carry the required amount, and eventually make a suggestion on other survival gear which will fit there easily "just in case".