To borrow from a friend's Closet you must have your Covet Fashion game linked to Facebook and have invited friends who have also linked their Facebook to Covet Fashion.

To Borrow from linked friend's Closets:
While styling for a Challenge, tap the item Type you wish to use and then tap the Borrow tab.
Then tap the "No Friend Selected" bar to choose a friend to borrow from.

To return to your own Closet, tap the Owned tab.
 
Please note that you may only borrow from the same Facebook friend once every 4 days!
You will be able to find the friends you have recently borrowed from under your Unavailable Friends list when trying to borrow items. You will also be able to see how many days and hours are left until you can borrow from them once again!