If, and that's a BIG "if", the foriegn entity is treated as a partnership for US tax purposes, your client will be subject to SECA as a partner. In the event the entity is classified as an association (i.e. a corporation) - more on that below - your client should then be treated as an employee and, consequently, not be subject to either SECA or FICA.
You should note that the rules of foreign entity classification under §7701 work a bit differently compared those applicable to domestic entities. Much of that is predicated on whether one or more of the partners do not have a limited liability. Otherwise, CTB election must be made for the entity to be treated as a partnership for US tax purposes or the entity would be classified as an association by default, which means it could be a CFC and there could be implications in relation to Subpart F, GILTI, etc.
The pros and cons of making the CTB election should have been weighed out at the time of formation as electing to change the classification at a later time could trigger recognition events.
Still an AllStar