If you can't trust your farrier to cut off that simple flap, get a new farrier. Actually, the pic you posted doesn't impress me much in the first place, but take that with a grain of salt since I don't know the history and you did not ask for a critique, therefore didn't provide critique quality pics.
It looks to me like it is just shedding. Yes, a large chunk, but I see a line where I think it ends and from time to time, this can happen. Again, without history it's hard to know, but sometimes big changes in the hoof can result in big shedding changes that were already on their way anyway. Personally, I don't let flaps like that hang about because I don't like them to get ripped off accidentally, but I have been trimming hooves for several years which makes quite a difference.
When is the farrier due to come out again? Maybe if he is in the area, he would be willing to detour just to cut that off for you... worth asking anyway.