How good of a friend?
If this is a person you see regularly and have access to her computer, you can sign in to your Steam account on her computer and enable Steam Sharing. She can then sign in to her Steam account and access your copy of Civ (and anything else you own) anytime you're not playing. (Note that because Civ is a "SteamPlay" game, it does not matter whether you believe you bought the "PC" or "Mac" version or whether she is running OS X or Windows; you are considered to own both and she can access both.)
If the person is too far away, then you can workaround this by changing & sharing your Steam password or using a remote access service like GoToMyPC - but be aware that in either case, for at least a brief time she would have direct access to your Steam account and be able to buy anything she wants and "gift" it to herself, so long as your Steam Wallet and/or saved credit card has money on it. So this is only recommended for the best of friends.
