Ok, so you missed the bit
ISBoxer usually configures WOW to use macros for Follow/Assist rather than other games that need to do Target character + Follow/Assist command, so this action won’t work unless you have configured Variable Keystrokes for your character set. If you did not configure the Variable Keystrokes for your WOW character set, then you will need to add a Named World of Warcraft Macro Action instead.
What this means is that your Group Target mapped keys should be using a Named World of Warcraft Macro action, rather than the variable keystroke action (unless you can't use the ISBoxer Addon for some reason, at which point you have bigger issues).
Anyway, to fix, delete the actions from the Step 1 of the Group Target X mapped keys, and replace them with a Keystroke Action -> Named World of Warcraft action, and select the "/targetexact {FTL}" option from the drop down.
it will look something like this.
Now, take note. The section for "Add Optional Modifiers" needs to be filled out. For each Slot number the modifiers will be different, although ISBoxer does set them up in a standard order for your character sets, so you can set it once, and it should work with all your Quick Setup Wizard generated character sets (for WoW anyway).
To find out what the optional modifiers should be you can look up each slot number by selecting the Character Set in the top left and then the Slot numbers in the lower right.
My check says that
Slot 1 = Left Shift
Slot 2 = Right Shift
Slot 3 = Left Ctrl
Slot 4 = Left Alt
Slot 5 = Left Ctrl + Left Shift.
So set those optional modifiers on the appropriate "GroupHeal Target Slot X" mapped key when you change to the Named World of Warcraft macro action, and it should be good.
If this is not the issue, then I can only think that your healer does not have anything bound in game to the 1 or the 2 key.