One thing you might try is asking them to show you an example of a great photo that hasn't been "'shopped". If you define it as any photo that wasn't exactly as it appeared when the shutter was actuated, they probably can't. Just as the decisions made in the darkroom had a great effect upon the finished product in the film era, the decisions made at the computer monitor in the digital era do the same thing.
Art critics have called Ansel Adams' photograph of "Moonrise, Hernandez, New Mexico" one of the greatest photos ever. Art historian H. W. Janson called this photo "a perfect marriage of straight and pure photography". Yet if you examine some of the over 900 prints that Adams produced from the negative himself it is very clear that over the course of more than two decades he produced a series with a remarkable range of variation. Adams explored the relationship between the various elements in the scene until he finally seemed to have found what he was looking for. The prints we now view as "definitive" did not appear until the 1960's. Adams snapped the image in 1941.
If your friends are not aware of the history of photographic technology there probably isn't a whole lot you can say that will change their minds. A picture that has been "photoshopped" is viewed as less than legitimate by the uninformed masses. Many people today will view a photo and instantly label it as "'shopped" so they can refuse to accept whatever statement the photo is making, deny the historical event it depicts, or reject the premise that such a photo could be produced without "photoshopping" it if they can't understand how it was taken.
In the end the only person you must satisfy as an artist is yourself. Did the final product realize the vision you had when you took the shot?
Welcome to the lonely world of the "Artist"!
Artnet.com article about the history and current market values of "Moonrise, Hernandez, New Mexico"
Wikipedia article for "Moonrise, Hernandez, New Mexico"
A photo of Adams late in life posing in front of two prints of "Moonrise..."