mailto code for php

here is a website http://www.business-broker.co.nz/ under "tell a friend" is the page i need help. There is a "your email" box where you get a reply saying thanks for telling your friend about us msg which is ok but where we have problem is the your "friends email" box where we want a mailto command to send to the friend. We have no luck there could you please let us know the code we should use and where to use it if possible please it would help my sanity very much and where does it go is it in the .lib or the .php file thanks for the help marc gander agent98@ihug.co.nz