In taking proceedings for the building not being p.d. solely on the basis that it might not be entirely within the curtilage, the planning enforcement department would be getting involved in a boundary dispute.
That is now unavoidable for them. And a Mk1 eyeball showing the building crossing what is "obviously" the boundary is enough to show that it is not within the curtilage of the neighbour
unless he can prove that it is.
They either deem it PD or not. "Not" would not of course automatically lead to any enforcement, but if they receive a complaint about the building they
have to come to some kind of judgement. "We don't know", "We don't care", "None of our business" - none of those are acceptable. If they receive a complaint that a building was put up without PP, what else is open to them except one of:
"It did have PP"
"It did not have PP but did not need it because it was PD"
"It did not have PP and did need it because it was not PD"
I really don't see how they can either just ignore the complaint or say it's none of their business.
They could then be accused by other council-tax payers of acting ultra vires and spending rates on what are private issues.
Building without planning permission is not a private issue.
Yes, the local rag can hough and puff, and people can complain to the chief executive. But all the enforcement officer has to say to the chief executive is that there could be a serious possibility of losing an enforcement appeal - and be liable for costs - and he will confirm the decision not to act.
If the OP's friend can prove that the building is on his land then there is no danger of losing an appeal. It's not a question of damages or compensation for the loss of the land, it's not a question of the guy trying to get an injunction to force the neighbour to remove the building, it is a simple question - is it PD or not.
PS; we also have to bear in mind that government guidance on enforcement states that; 'it may not be appropriate for a minor or technical breach where no harm has occurred'. Councils have to go by these rules.
Theft of land is not a minor or technical breach.
But I've already said that even if they agree that it is in breach of the planning regulations they could well decide to do nothing. But that's when the publicity campaign starts. And that's when adding the fact that the outbuilding has no planning permission is communicated to the estate agent(s) for them to tell prospective buyers.