I work in this area OP so just want to give you an idea of what you’ll want to see now it’s been reported.
Ideally an officer will come out to inspect the building and take measurements. If it isn’t substantially complete they’ll make inquiries about what it will look like/be used for. They may issue a PCN to get this information - failure to respond is a criminal offence.
They’ll then make their assessment of it and possibly seek their own legal advice. If they think that there’s no breach they’ll leave it there. If they think there is a breach but it’s not expedient to rectify it they’ll leave it there.
If they think there’s a breach which is expedient to rectify, but it could be regularised via a planning permission, they will advise your neighbour to apply for retrospective planning permission and you’ll have an opportunity to object during that formal process. If retrospective planning permission is refused and your neighbour doesn’t appeal, the LPA should serve an enforcement notice immediately.
If they think it’s a breach which is expedient to rectify and they don’t think it could be regularised by a planning permission they should move straight to an enforcement notice.
An enforcement notice should require removal of the building and that the land is returned to its previous state and the rubble cleared (within a suitable timeframe). If your neighbour doesn’t comply the LPA can enforce the notice via the magistrates court and in the worst case they can go in and remove it themselves.
Your neighbour would have the opportunity to appeal an enforcement notice to the Secretary of State.
The law changed last year so now the time limit for enforcement action relating to operational development is 10 years (whereas this previously would have been 4 years). In our LPA it’s common for enforcement notices to be going out running up to the deadline. It’s a resourcing issue.
Getting local councillors involved can be helpful in that they will put pressure on the officers to look into it quickly, but it won’t change the result if the view is that there is no breach, or there is a breach but it’s not expedient to take enforcement action.
All in all the process can be drawn out over a number of years, depending on various factors.