OS is irrelevant. You need it to be saved in version AE 11.
That means somebody needs to open it in CC 2014 (version 13) and choose to Save As CC (version 12). Then somebody needs to open in in CC (version 12) and choose to Save As CS6 (version 11).
See the second part of this: opening After Effects projects from previous versions and saving back to previous versions
And, if somebody doesn't have the old versions around to save it from version 12 to version 11, they can install them (it doesn't take very long): How to Find and Install Previous Version of Adobe Apps in CC 2015 | Adobe Customer Care Team