Just because the foot is flat (arch collapsed) does not mean you have a right to try and lift it !
This is a perfect example of a foot that is troubled. It is a rigid flat foot deformity. This acquired over a long period of time. Sometimes tibialis posterior insufficiency over time finally gives way to an incompetent tib posterior, with eventual arch gradual collapse into a pes planus flat deformity, and then time takes its effect to contracture and shorten tissue and arthritic change makes it permanent.
This arch will no longer lift, it is a rigid pes planus. IT will not tolerate an orthotic, SO DO NOT PRESCRIBE ONE ! Even a mild orthotic lift will feel like a golf ball under this arch.
And, to take this one step further, a rockered shoe is, in part, the right idea, but not when the foot does not sagittally toe off. This foot is permanently locked into a full limb external rotation because of hip arthritic change. The point is that his foot progression angle is 45 degrees++, and the rocker will not work if it cannot rocker in the sagittal plane.
This guy wanted an orthotic, and i would not give it to him, and you shouldn't either. He will wear it for 1 minute and throw it away.
Shawn Allen, the other gait guy