There really isn't a way in Facebook to do this while on a website. We do have a way to allow for friend tagging when using our Open Graph Action popups (where you can write a message about something you're doing and where you're at). So, it is possible to fetch the current user's friends and show a drop down. To test what I'm talking about, go to
facebook.sourcecoast.com , login, click into a recipe and hit the "I ate" button.
However, that's a very specific field, so it's easy to monitor for a user's friends name being input there.. that's the expectation.
If you can tell me more about how you would want the tagging to work, and how users are entering their friends names, that may help us understand more. Are your users actually writing content articles on your site? Would you want the tagging to be a drop-down as they type someone's name, or just try to automatically detect a name and link it to Facebook? Need more information, but I'll be honest, it'd be very difficult to do what Facebook does on their site (scanning each character entered to see if it may be a friend).
Thanks,
Alex