Personally speaking I think that it not the 'done' thing. It is a form of fraud which in itself = dishonesty. Yes he may be a dab hand with Photoshop so why not get out there and take pictures of his own and then use photoshop?
Photoshop images are one of the reasons I am drifting further and further away from digital imagery. I don't think it is as honest or as skilful as traditional photography and nor will it ever be. There is a whole world of difference in making a picture sitting at a desk with an image and fiddling about here and there and elsewhere using adobe which is someone else's technology. And then looking at a scene and visualising what you can do with it in the darkroom using the skills you have in your head and hands and to a degree your heart. My ex wife thought that digital imagery was a subject that 'had no soul'. It was one of the things we actually agreed upon.
I also look upon digital images and comparing them with an item of furniture made by a master craftsman and a piece of furniture that is mass produced in some factory somewhere. The latter will never have the feel of quality of the former