This is really serious stuff. You can manage it, he can recover, but you have to be faithful about the care involved. Beings from Florida the vets and farriers down there will tell you Floridians invented seedy toe.(white line) Its all over the place and yes it has crossed my path too. Its easy for a fungus to work it's way up there and leave your horse with a hollow foot especially if your horse has been a product of neglect in the past nutritionally and by bad hoof trims. An x ray should have been done first before the farrier did his resection. Once opened, you can take a syringe (without the needle) and shoot something good up inside to flush. I used merthoiine. but the white lightning gel is very good too. Then you have to wrap it up and keep it clean. Keep in mind its wide open! . I used vet wrap with duct tape and keep the horse away from mud or anything that could get his feet wet. That's key. Then unwrap and clean it every other day but you have to be religious about it to get anywhere. Also the horse should have been put on Farriers Formula or Biotin and don't forget a good feed program.. I think in this matter your farrier put the cart before the horse so back track. I don't really remember because it was long ago but i think I may have keep it unwrapped while in a stall with clean shavings. Ask your vet about that. Good luck with this.
