If the whole of the plot of land owned by the lady who sold it to him was registered at the LR, and this lady has recently sold the whole of this parcel of land to a third person, then you can see the newcomer's point - he and his solicitor have signed contracts to purchase the whole of the parcel of land at x price, the General Boundaries of the plot will be shown on Title Plan that he has bought, but then some neighbour turns up later to announce that the seller had previously sold part of the plot much earlier.
He and the original seller have both gotten themselves into this mess, thus it is unlikely the new-comer will want to help much. There is a question on the land transfer document that asks 'is this transfer the whole of registered plot' and she must have answered yes to that - which was incorrect because she had previously done a deal on part of the plot earlier.
How many square metres are we talking about?, has it been fenced off into his plot (such that it should have been blindingly obvious that the land described in the Title Plan was much larhger than that on the ground)?
What does the seller have to say about this? - why didn't she declare at the time of sale to the newcomer of her whole plot that she had previously done a deal to sell a smaller parcel?
What is absolutely clear about these sort of situations is that individuals should strive to get resolution without recourse to legal action because the costs are huge (as your friend has been quoted).
Depending on the answers to the above depends on the strength of your friends case, but his primary place to discuss this matter is with the lady who sold him the land, not the newcomer (who on the face of what you have said so far has done nothing wrong).