This reminds me of a story I read a few years ago where a highway contractor did not follow the specified plans, and effectively made a piece of private property part of the highway.
The property owner complained to the contractor, asking for the road layout to be modified such that it did not take any of his private property.
But the contractor refused, pointing out that he had had the works signed off by the local council and had the relevant official form (whatever it was) to prove it.
The land owner then complained to the local council, who did not want to know about the problem, having signed off the work as being completed as required.
The land owner then complained to the local government ombudsman re the council’s behaviour, which resulted in loss of his land – the last a recall is that the landowner was not having much luck with his complaint to the ombudsman.
All the above happened in England (as far as I recall), so things could be different in Scotland. But nevertheless I would expect Scotland to have some similar process whereby highway works are officially signed off, and given the time period, that those works had indeed been signed off in some way.
I suggest your friend conducts a search on the topic to find out what the process is in relation to this in Scotland. It may very well be that the works were signed off, and if so, your friend could point this out to the local council – and if they now want to make changes, that will be at their expense.
If I were in your friend’s position, I would certainly not be willing to pay anything towards remedial work (even if the original error was my fault), given the length of time that has elapsed since the work was completed.