Well the easiest way to do it (and the best way) would be to use mIRC's built in notify list.
/notify the_users_nick
ON ^*:NOTIFY:{
if ($nick == the_users_nick) {
.echo -a Hey, he's online!
haltdef
}
}
ON ^*:UNOTIFY:{
if ($nick == the_users_nick) {
.echo -a He's not online
haltdef
}
}
