Tora`s answer is completely wrong. You simply cannot scale a physical boundary that exists on the ground by reference to the little red lines that the LR draw on the title plan. Well, you can, it is called determining the exact boundary and involves parties agreeing to a plan drawn up and paid for by a chartered surveyor. Google General Boundaries, Tora, if you don't believe me.
What has happened here is the physical boundary appears to have been shifted. If their was a masonry wall on the boundary that the builders knocked down, the Party Wall Act should have applied, but it is too late to invoke that now.
Photos are good, as The Builder says, but hindsight is a wonderful thing, and legal action would be ridiculously expensive over what is an annoying matter of a couple of square metres.
In the case study of my answer to the question below this one, the issue came to light because the LR raised it as an alignment discrepancy. I do not believe you can go to the LR for adjudication on this - they won't play ball.
It is a civil matter and nothing to do with planning consent or building regs.
The best bet your friend has to resolving it is to remind the owner that it will be very difficult to sell after development, as no-one is going to buy it with an outstanding dispute in progress. Time, when you are paying a mortgage with no income an an empty property, might focus their mind a bit.
Try transferring a small strip for a payment of suitable size?