Quality of hoof material is basically originated in good balanced nutrition and proper exercise (and therefore hoof stimulation). There are no hoof dressings or topicals that will "fix it". You can feed biotin by the bucket and if you do not have proper lysine in your horse's diet, it will not be helpful. Lysine is a limiting amino acid that is required in recommended minimum daily amount (23 g) to allow many other normal processes to occur, including proper utilization of proteins. Hoof health requires lysine, biotin, methionine and zinc. Check your feed source and make sure it is balanced. All in all, the percent of protein in your feed (say 12%) doesn't really mean too much. 12% feeds DO NOT have 12% crude protein (maybe about 8-10% only in reality). You need to know the grams of crude protein, not the percentage. Horses need digestible protein (.60 g) and crude protein (1.26 g) per kg of total weight/day. Crude protein in feed is directly related to the nitrogen content and labeling which sometimes has to meet a standard (not too many standards required for animal feeds unfortunately), they are usually incorrect as far as "meaning what they say". Sorry, didn't mean to go all chemical. Anyway, when correcting a nutritional problem, it will take 8-12 months to see results. There is no quick solution.
I would be curious as to why your horse has a crack? Are her hoof walls uniform in thickness all the way around? Has she been barefoot for a long time? I would be curious as to how she is trimmed. A proper barefoot trim can withstand almost anything, especially if the horse is getting consistent exercise. If the secondary or primary lamina is separating, the walls are too long - she is tearing them with her movement, plain and simple (if she is otherwise healthy). Until this mechnical problem is corrected, she will not be able to grow in a new connected wall. I would just make sure she is trimmed properly. I would not put shoes on her. Shoes will cause more damage and unnatural stress and pain. Not to be disparaging about your farrier, OK. Also, you didn't say where the cracks are ? quarters, toe? With proper trimming, she should be fine in sand or any other terrain (relative to wall cracks). Putting shoes on her will weaken her feet and they will never be as healthy as they could be.
In the long run, nutrition is the key. Good nutrition does not require supplements. With balanced nutrition, supplements create an imbalance and the problems that come with them. It will take time. Hope you can be patient for her sake. Get her properly trimmed and let her grow in a new connected wall. It can be scary to remove old damaged wall if you are unsure of what and how to do it and too often shoes are thought to be the ideal solution. It isn't. Maybe consult with a good barefoot person is you can find one in your area. Best of luck to you with your horse.
Just wanted to add one more idea. Dependent upon the condition of her walls and the amount of separation present, again-someone has to make a definitive call here, if the separated wall is trimmed as well as it can be (and a lot of damaged wall should be removed to prevent additional damage and remove the stress from the separating wall - but without making the horse sore or lame - it can be done), I would cast her front feet if you want to still get some work done with her. It will just protect her and allow her to work at some level. The casts will have to be replaced every two weeks to keep the walls properly maintained until new connected wall grows in. Again, you will need someone who is knowledgeable in this technique and it is not inexpensive, especially at twice monthly. But I think it is worth it if the case is appropriate. Again, wish you the best.