LOL!
Some folks will not listen, period. The fact that virtually all canning instructions indicate the jars should be upright after all many decades of research, in addition to real-life experience by millions of people, doesn't have an effect.
Another factor in canning with other-than-recommended methods is that if one were to place a quart jar on its side in a smaller (shallower) pan, it has less water to heat. The larger the heated surface area of the same amount of water, the faster it will reach boiling, proportionally affecting the jar contents' temperature. With that less water, the boiling temperature is reached sooner. In the case of tomatoes or peaches which have a low processing time, it might not allow the inside temp of the jar contents to reach the desired/required temperature. Remember, canning instructions are based on the entire directions, not just a portion of them. In a typical hot water bath canner, there is quite a lot of water that must be heated, during which time, the jar contents are also heated. At one time, "baking" product in the oven was used. After all, in theory, it is just as good, right? Well, that method has been removed from all approved canning methods, I suspect for much the same reason (interior temperature of the jar contents). (That one never made sense to me anyway.)
Some folks think the only dangers are the ones they can see at the moment. We tell young children not to play in the street because they might get hit by a car . . . but they see no cars coming so our instructions make no sense to them. However, we, as adults, know that sooner or later cars will be driving on that street, hence the instructions.
You might suggest to him to invest in a pan four inches taller than pint jars and can in pints, or use half-pints in his current pans.
Glenna