Hi,

You need to place the Send to friend web part on some page. You can set some of the default web part properties - Path, Show for document types, Document types, etc. you can find more information about the properties by clicking the Documentation link in the upper right corner of the dialog window.
Then you need to set the transformation property. You can create custom transformation (http://devnet.kentico.com/docs/devguide/writing_transformations.htm) and in this transformation you will resolve the link to that document.

In the Email tempalte field there are two macros by default:
{%Message%} - this will be resolved into the message which was added by the sender.
{%Document%} - this will be resolved with the transformation you have specified.

For masonc:
If you want to have the link and the form opened after the click, you will need to customize the behavior of the web part to be opened in a new window.

You can take a look how it is working now in the sample E-commerce site in the Products section.

Best Regards,
Juraj Ondrus