This is not strictly true, the farmer/ landowner is obviously allowed to shoot any legal quarry or shoot clays or targets on his land, the fact that someone else does not like this is not a breach of the peace, it is only if he were to make threats or shoot in the direction of the other party that he would be considered to be committing an offence.
The 50 yard rule only applies to public acces ie centre of road or footpath.
The question of suitability is also false, every UK citizen has the right to have a shotgun, it is for the police to find reason why they should not grant a license ( previous custodial sentence, medical issues, previous concerns )
Remember my point re 25 days- this applies to all the fields your friend has not just the one you are using. Your farmer may report a breach to planning.