I'm certainly not a PHP expert, and I'm not familiar with this extension per se, but it would seem to me that detecting whether the current version was also sighted would be as easy as comparing revision ids. If the current ID and the currently sighted ID are identical, don't show the link.
Maybe I'm over-simplifying, and sighted revisions are not marked according to revision number, like I said I'm not familiar with this code.
--Andrew Whitworth