If you are wanting your friend's site to link to yours, you will need to ask your friend to do that.
If you are wanting to link to your friend's site:
<a href="http://yourfriendssite.com">Some text or an image</a>
Hope this helps!
You ask your friend to link to your website. He does it.
Perhaps I don't understand what you are looking to do.
To link to someone else's website, simply add a link to either some text describing or naming that website or to a picture. I do this all of the time in websites I create on links pages:
Here is an example: http://www.crittersittersct.com/links.html