Assuming your script works (haven't tested the actual reversing part of it), this should do what you want:

on ^*:text:*:#test_bot: {
  if (%backwards == on && $nick == PersonY) {
    var %c $strip($1-),%a $len(%c)
    while %a >= 1 {
      var %b %b $+ $replace($mid(%c,%a,1),$chr(32),$str($chr(32),2))
      dec %a
  echo -ambflt < $+ $nick $+ > %b | haltdef

Note that depending on how you have your <nick> format set up, you may also want to add the nick's mode (such as @) and the color of the nick in there.

Don't forget to change "PersonY" in the script to the nick you want this to work on. If you want it to work for everyone, remove that part of the IF check. If you have multiple people, you can make it work for more by changing the IF as well.

This also assumes you're using the same menu part of what you posted to set the backwards on/off. I didn't bother copying it into here since there wouldn't be a change.

Invision Support
#Invision on