Watermarked images from stock sites cannot be used in final designs. They are, however fine, for prepublishing designs, to allow customers to decide on the images to use. But even for internal Powerpoint presentations before a limited public, you would need to license.
You don't need to state that explicitly, if it is stated, it's just to be clear for the amateurs.
:copyright: infringements can be costly.
Edit: I was quite sure that there was a provision in the licensing terms for use of watermarked assets. So I searched for that and here we are:
IMHO: the advice of that guy is highly dangerous and exposes the customer and his company to criminal investigation.
ABAMBO | Hard- and Software Engineer | Photographer