Although he can do nothing, he should do something. He can consent to the work and that is all there is to it. Work can start immediately. Both parties should keep the documentation and file it safely. In the vast majority of cases where simple work is proposed, there is no reason for a dispute to arise and consent can be given safely.
Alternatively he can dissent from the Notice. He should not do this just to be awkward or to make the Building Owner jump through the hoop nor to frustrate the Building Owner’s proposals. The Planning process is the proper place for such objections. Having dissented, the parties will need to appoint a surveyor to settle the matters that are in dispute.
If he does nothing for fourteen days, then in the case of Party Structure Notices and Notices of Adjacent Excavation (also referred to as Three Metre/Six Metre Notices) it acts automatically as a deemed dissent from the Notice. The parties will then need to appoint a surveyor to settle the matter