it appears that right now, if javascript is disabled, the URL of the iframe contains "TB_iframe=true" in the query string, but that value does not appear if javascript is enabled... so, on the 'email a friend' page, check if that value exists, and if it does not, pass some value through when the form is submitted so you know on the resulting page (the one that currently has the redirect) that the form was submitted via the thickbox iframe instead of a "normal" page (in which case you want the redirect). then on that redirect page, change the text to some sort of "thank you" text, and add this javascript to the page to close the thickbox window:

top.TB_remove();

that will close the thickbox window for you. then the only other decision is what to do with the now hidden thickbox window (since it will be displayed again if the user clicks the "email a friend" link without reloading the page)... do you keep it as a "thank you" page, or do you redirect page back to the email form... I'd probably redirect to the email form, but either option works.

instead of calling top.TB_remove(); outright, add some logic to check for either an element that exists on the "Thank You" page but not on the "Error" that you can get to by document.getElementById, or maybe the page titles are different you can check for that (document.getElementsByTagName("title")[0].innerHTML), or at the very least, get the innerHTML of whatever element contains the "Thank You" or "Error" text, and check if that text contains "Error" (use indexOf() or a regex... i'd go with the indexOf cuz it's easier)... if it does contain "Error", don't close thickbox, and if it doesn't, then close it.