cracking on the edges of the hoof wall would say to me that the farrier is not taking enough wall off with each trim and the excess is cracking away when in contact with hard/rough surfaces, due to peripheral overloading. there could also be the added issue of not enough nutrition in the diet and that is showing up in weak hoof wall that cracks easily. do you have her on any supplements?
both my old gelding (had him 10 years) and my current mare (had her just over 4 years now) stayed barefoot the whole time i had/have both and my farrier has been the same for both (i was so grateful after a 12 year horse hiatus to find out he was still doing hooves!), and i have never had cracking happen on either horse's bare feet. both were/are on a 8 week trim schedule and live/ride on the same varied ground - dirt arena, grass/dirt/hard packed ground for 24/7 turnout (same barn for both horses). no supplements, just hay or grass depending on season, and well water that is quite high in iron and other minerals.
pics of her hooves would be very good.