The ZED agreement REQUIRES that carriers bilaterally accept EMPLOYEE, SPOUSE (LEGAL SPOUSE), and CHILD. There are several other eligibility categories but they are OPTIONl (i.e., airlines may -- or may not -- agree bilaterally to include them).

Travel companion (regardless of whether they are "in lieu of spouse" at your own airline) fall into the OPTIONAL category. This category is often included in alliance agreements (e.g., oneworld, SkyTeam, Star) but generally not outside of that. Although, there are a few European carriers known to include them routinely.

Ultimately, it all boils down to your airline's bilateral agreements