well if you wanted to use the $nick(#,nick_here).idle then you could do something like this i guess...
client that is running the code needs to be +o in the chan an users need to be either +o or +h to use it
This is only an example:

on $@*:text:/^@idle\s(\S+)$/iS:#:{ 
  if (!%f && $nick(#,$nick,oh)) { inc -u5 %f
    msg # $iif($regml(1) ison #,$+($v1,'s) Idle time in # is$&
      $duration($nick(#,$v1).idle,1),Sorry $v1 is not in this room) 
  }
}

or if you want to do the whois way you could do this..
example only

on $@*:text:/^@idle\s(\S+)$/iS:#:{ 
  if (!%f && $nick(#,$nick,oh)) { set -u5 %f #
    if ($regml(1) ison #) {
      .enable #idle
      whois $v1
    }
    else { msg # Sorry but $v1 is not on this channel. }
  }
}
#idle off
raw *:*: {
  if ($numeric = 317) {
    msg %f $2's idle time is $duration($3,1) 
    haltdef
    .disable #idle
  }
  else { haltdef } 
}
#idle end