Agree with Bill.  What your client needs is a Certificate of Coverage (CoC) from the Greek fund to which your client makes contributions.

Unlike employees who are seconded to Greece on assignment, where the CoC just needs to be kept on file in case of audit, self-employed individuals must attach a copy of the CoC (or the statement from SSA as Bill mentioned in the unlikely event your client is not able to obtain a CoC) to the 1040 each year.

Having said that, it is not uncommon for clients who operate businesses overseas to mistake their employment status for US tax purposes.  (Well, that's probably true for domestic shareholder-employee of S-corp too but even more so for foreign businesses.).  This is because US tax treatment is not driven by how a particular entity type organized under foreign law is treated for tax or legal purposes in that particular jurisdiction and the IRC's classification regulations for foreign entities are slightly different from those for domestic ones.  You will, therefore, need to determine whether the foreign entity type of that particular country is classified a per se corporation based on IRC regulations, the default classification depending on whether there's one or more owners as well as whether each has a limited liability, and whether CTB election was made.  For example, if your client's foreign entity type is not a per se foreign corporation, your client is the sole owner, and your client has only a limited liability, the default classification is generally a corporation (and not a DRE) unless a CTB election is made.

Foreign entity and it's classification also create a different set of issues and compliance requirements that need to be considered.  These include PFIC, Subpart F, and GILTI, partly dependent on their types of income and business activities.