Since before they got him he's had a pretty bad crack in the front of one front hoof that goes 3/4 of the way up. I suggested their farrier rasp a perpindicular line at the top ....He has had flat feet since they got him, so they've kept him in shoes to protect his sole.
Firstly, what she said above.
Regarding the crack, this is an indication of undue pressure on the walls. It may also have come about because of lami/separation & perpetuated by infection. Treating the infection, ensuring diet & nutrition is good, and trimming to relieve the unhealthy forces on the walls is what is needed to allow it to grow out. Cutting lines above the crack can work(if the hoof is cut thru the entire wall) to stop that crack, but because it doesn't address the actual causes, it generally means the cut hoof may tear elsewhere, or it's only temporary. Also cutting through the entire hoof wall opens the foot up to further risk of infection & therefore damage.
Regarding shoes protecting his soles, they don't, unless he also has pads, for a start. But laminitis aside, shod horses are frequently flat footed, *because* the soles are taken out of commission. Therefore the walls are forced to support the entire horse. When a horse is effectively hanging from his laminae in this way, if the laminae are at all weak, the foot will 'drop' in the capsule. The sole is effectively 'bottoming out' to get some much needed support. What he needs is to be able to use his soles properly - to actually walk on the bottom of his feet, as he's evolved to. Then the soles & frogs can start to thicken & strengthen, and the walls can be relieved so the laminar connections can become strong. Obviously he's not in a position to do this without protection yet, and I agree with Barefoot that boots &/or pads are the best option.
hoofrehab.com is one great site that should help you & your friend get educated on hoof function & principles necessary for soundness. She may be 'against a natural approach' and may have good reasons, but then again, it might be lack of knowledge about it, so perhaps you can encourage her to learn about it & then weigh up the pros & cons to come to an informed decision.