Many people who are "recoil shy" have not yet felt recoil from any gun. As an instructor I have found a number of people who mistakenly associate size of the gun with force of the recoil. I had one man who understood some basic physics, and merely telling him of the inverse relationship between gun size (mass) and felt recoil was sufficient for a head-slap, and a realization that the "big" revolver I was starting him with (K-38) would not recoil as much as the snub J-frame he had purchased, and was concerned with.
Another student had to watch me demonstrate different size handguns and different calibers before she would even pick up a Hi Standard 103 that I hoped would eventually lead her down to the little 22 popgun that her husband had purchased for her to use as a purse-gun (his term, not mine).
There is no need to explain Einstein and Relativity. Just let the student handle different size guns, and see different caliber ammo, and then start them out on decent size gun, mild caliber, to get started on learning the basics.
PS: Be sure you explain and demonstrate safe gun handling while you are doing it!