The page actually gives this warning:
Info: Prima effettua il login
(i believe it says: "You must Login First" ).
But since the Joomla messages stay outside of the "content" section in your website and this popup shows only the content section you see blank page. (popup lightbox does not show the standalone page, but it shows only what is inside the content section)
See the difference between this page:
www.cosecool.com/it/home/fiorifrancesi/decolte-con-plateau2012-12-12-09-51-20-detail/recommend.html?tmpl=component (this is popup lightbox version)
www.cosecool.com/it/home/fiorifrancesi/decolte-con-plateau2012-12-12-09-51-20-detail/recommend.html (it is the normal version of the same page)
But in your case ilightbox version doesn't show the "error message" at all. it was supposed to show "Info: Prima effettua il login" in the lightbox version though.
There is an another option in the VM Configuration which lets visitor to send the product to "friend" without being logged in, you might try to enable that option as well;
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Regards.