There are lots of options for ‘similar to issue’ styles available. A google for “oxford shoe” will yeild results at varying price ranges. I’ve had a number of cadets struggle to get DMS shoes in their small size, so they opt for a £40 pair of oxford style civie shoes.
Though, where the issue is fitting orthotic insoles I would have thought that, ironically, the issue shoes - coming in three different widths per shoe size - might actually provide the most combinations. If the wearer is a Size 9 for example, and a 9L won’t fit with the insoles, would a 10M perhaps work?
I go up in boots for that very reason. It certainly can be tricky, but there are definitely options available to them. Nobody should have a problem with it.