mirc hotlinks words in the buffer by checking that:
a) its a url
b) the word is in the nicklist
if it doesn't match a or b, mIRC has no way of knowing the text is even a "nickname". mIRC doesn't save meta-info along with the lines it echos to the channel buffer to explain what it just echo'd-- once it displays the part message, its regular text just like everything else.
That said, you can freely script your own ON HOTLINK event and keep track of nicknames on your own.
on *:PART:#:hadd -m nicklist $+($cid,$nick) $true
on *:QUIT:#:if ($hget(nicklist)) hdel nicklist $nick
on ^*:HOTLINK:*:#:if (!$hget(nicklist,$cid $+ $remove($1,$chr(44),.))) halt
on *:HOTLINK:*:*:if ($hget(nicklist,$cid $+ $remove($1,$chr(44),.))) whois $1
Of course you can't get a popup menu from that until Khaled implements popups for hotlinks, though there are dlls that can make this possible.