BS ~ !!!
As long as he is doing it for his own personal use, and enjoyment, he's not hurting anything or anybody.
Now if he's going to upload it to a site that's a different avenue.
I've done all kinds of Tweaks and customizations to aircraft and scenery and other stuff I've got from other aircraft / scenery designers,,,,, for "my own personal enjoyment" and use,,,,
and the best part, I can do it without hurting anybody else or breaking any copyright rules !!!!!!!!!!!
I've tweaked aircraft model files, scenery files, re'painted hundreds of Aircraft of somebody elses, even PAYWARE Aircraft and scenery.
and I did NOT check or attempt to contact anybody about it !!!
So, I changed the paint, on some aircraft, changed or added the reflection textures on a plane, swapped texture files , added or deleted a file on a scenery
The only thing "Un'Cool", is those who instantly & immediately cut down someone else for learning and experimenting, which is just how many of the other developers learned their skills.
Thousands of other flight simmers are always customizing their aircraft, scenery mostly for "their own use & enjoyment" !
Now IF he plans on uploading his whatever , of somebody elses creation, that's a different story .