There are two parts to this process. First, you must enable customers to publish their wish list with an RSS feed. Then, you must encourage the customer to send the RSS feed to his/her friends.
However, the question then becomes, "Why would anyone subscribe to a customer's RSS feed for their wish list?"
A customer can share his/her wish list. In the following screenshot, notice the Share Wishlist button:
When a customer clicks this button, the customer is taken to a page where he/she can send a message to the person with whom he/she wants to share the wishlist:
Notice the checkbox that ...