OK, apparently the previous messages didn't take into account permissions or SteamPlay cloud sync.
First thing you want to do is turn off SteamPlay cloud sync on your account (the one you are copying to)
Then you need to copy the game files (mentioned in the other answers). If you don't have read permission, from your account make sure to set that up on your friend's account.
After that you may need to change the permissions on the new save. (I'm not sure if finder's copy routine defaults to parent directory permissions or source directory permissions. It's best to check and change them if necessary.)
Finally you need to open the game with with the new save file, quit it, turn back on SteamPlay cloud sync, and then open and close the game again to sync the saves to SteamPlay.