The main thing is to replace "@Whois- $+ $2" (creating a window title containing the nick of the whois'd user) with "@Whois" (creating a general whois window)
Code:
raw 311:*:{ 
  if (!$window(@whois)) { Window -a @Whois 100 100 490 180 }
  aline -p @Whois 10Nick:4 $2 10Address:4 $3 $+ @ $+ $4 $+ 
  aline -p @Whois 10Real Name:4 $6 $+ 
  haltdef
}
raw 319:*:{ aline -p @Whois 10Channels:4 $3- $+  | haltdef }
raw 312:*:{ aline -p @Whois 10Server:4 $3 10Network:4 $4- | haltdef }
raw 301:*:{ aline -p @Whois 10Away msg:4 $3- | haltdef }
raw 307:*:{ aline -p @Whois 10NickServ:4 $2 10has identified for this nick. | haltdef }
raw 313:*:{ aline -p @Whois 10IrcOp:4 yes | haltdef }
raw 317:*:{ aline -p @Whois 10Idle Time:4 $duration($3) 10Signed on @4 $asctime($4,dd/mm/yy HH:nn) | haltdef }
raw 318:*:{ aline -p @Whois $chr(160) | haltdef }

menu @Whois { close : window -c @Whois }