What version of AE is your friend running? Have him go to Help>About and it'll tell him the exact version he's running.
This isn't a Mac vs. PC thing, this is a version of AE thing.
After Effects has no problem opening projects from either platform.
As Szalam has said, the issue here is that your friend is using an earlier version of After Effects than you. You're using After Effects version 13 (CC 2014).
If your friend is using version 12 (CC) than you can just save him a CC version of the project. You can save to the previous version of After Effects using the Save As command.
If your friend is using an earlier version than 12 (CC), you will need access to earlier versions of AE so that you can retro-save your project back to the version he requires.