Some of the other scripts on the page appear to have interferred with the script that Tell-A-Friend requires. Specifically, the mootools-1.2-core script appears to have overwritten the JSON function. I'm not sure how that script is used on the page, so I'm not sure what the best way to work around that is (the easiest way would, of course, be to remove that script from the page, but I don't know what that would break).
Hope that helps,