My friend, a jewelry designer, would like to give the necklace she's making for me to wear at the ceremony as her wedding present.  Is this unfair to my fiancé?

What a kind offer!  I can't imagine a more generous and meaningful gift.  How could your fiancé feel anything but touched by this gesture?  And he does benefit, albeit indirectly - your wedding budget now has a little more wiggle room.  Suggest to him that you take the sum you would have spent on jewelry and instead use it for something you'll both enjoy, such as a spa day for the two of you during your honeymoon.