Ouch! Doesn't look as bad as... it looks tho, to me, if you get what I mean
The loose section of hoof will need to be cut off ASAP to prevent further damage. I would think(& of course hope) not likely the case here, but it's possible with this sort of injury there could be damage/fracture to P3 &/or lateral cartilages, so I'd personally want some radiographs taken to rule it out/find out, if it were my horse. The corium(open meat) will need to be cared for to prevent infection - they may proscribe antibiotics & regular iodine apps or some such. **I would be considerate that continuing applications of iodine & such can damage live tissue, & being corium/laminae, can then ****** the growth of new horn material. Therefore I'd be inclined to use Manuka honey or some other antiseptic which is not harmful to live tissue.
Of course there are always going to be different opinions & approaches, so I would also caution you to learn what you can about the principles behind both different opinions, in order to make an informed decision, but I have quite an opposing opinion of what 'should' be done, compared to what Foxhunter describes their farrier would do... (not meaning to 'pick on' you Fox, just...
I believe that the entire ground surface of the hoof capsule should share the load of the horse, rather than the traditional idea that the walls should support the entire load. While healthy walls share the load, that's not their primary function & especially in cases of damage or sick feet, IME they are often best *relieved* of ground pressure. Therefore far from putting a shoe on, to peripherally load the foot further, I'd be cutting back/bevelling the wall strongly on that quarter to completely relieve it, until some healthy growth can come down. *It's also important the good balance of the entire hoof is considered & kept well maintained(not allowed to overgrow), as imbalance can easily put more pressure on surrounding areas(such as that heel) to further exacerbate the damaged bit.
Cutting or burning a line(or 'x' or whatever) across the top of a crack does nothing to stabilise it. Removing undue pressure from underneath will allow it to heal without the mechanical forces on it which may cause it to continue. Cutting a 'v' in the ground surface to take pressure off the crack won't be effective if the rest of the wall is left loaded. Instead of a 'v' I would be keeping that whole quarter well bevelled to near the lamellar line. *Once new horn has grown over the corium, being vigilant and proactive about preventing/treating any seedy toe will be vital.
Bracing the wall across a crack - such as Foxhunter describes with the farrier putting a nail through it - can be helpful/necessary, just that I'd choose to use a strap or plate type affair(such as some hoop iron screwed in place either side) rather than just a bent nail.
I would not advise using hoof goop at all either. Despite advertising of some products to the contrary,there is no evidence of any topical goop improving health or growth of the horn(& once it's grown, it's dead tissue, so like hair, nails, etc, you can't improve health with magic topical products, only make them look better.
The risk of damaging the external 'periople' layer of the hoof & making it permeable, and the risk of giving opportunistic infections a nicer place to party by sealing the hoof with grease are the main reasons why I would not advise topicals except occasionally(to make them prettier for shows or such
) & only on healthy feet. Nutrition and exercise(more/good hoof function) are what will improve & speed up horn growth & healing. I speculate that massaging something into the coronary border may help growth due to the massage, not the product.