How do I Horizontally Flip a layer in Photoshop using a script?

This might be easier.

flipLayer('v');  
//flipLayer('h');  
   
function flipLayer(direction) {  
switch (direction.toLowerCase()){  
    case 'h' : direction = 'Hrzn'; break;  
    case 'v' : direction = 'Vrtc'; break;  
    default : return;  
    }  
    var desc23 = new ActionDescriptor();  
    desc23.putEnumerated( charIDToTypeID('Axis'), charIDToTypeID('Ornt'), charIDToTypeID(direction) );  
 try{  
    executeAction( charIDToTypeID('Flip'), desc23, DialogModes.NO );  
 }catch(e){}  
}; 

This is from this thread..

https://forums.adobe.com/thread/932286

You can accomplish everything with ScriptingListener

this and for horizontal

// =======================================================

var idsetd = charIDToTypeID( "setd" );

    var desc22 = new ActionDescriptor();

    var idnull = charIDToTypeID( "null" );

        var ref9 = new ActionReference();

        var idChnl = charIDToTypeID( "Chnl" );

        var idfsel = charIDToTypeID( "fsel" );

        ref9.putProperty( idChnl, idfsel );

    desc22.putReference( idnull, ref9 );

    var idT = charIDToTypeID( "T   " );

    var idOrdn = charIDToTypeID( "Ordn" );

    var idAl = charIDToTypeID( "Al  " );

    desc22.putEnumerated( idT, idOrdn, idAl );

executeAction( idsetd, desc22, DialogModes.NO );

// =======================================================

var idTrnf = charIDToTypeID( "Trnf" );

    var desc23 = new ActionDescriptor();

    var idnull = charIDToTypeID( "null" );

        var ref10 = new ActionReference();

        var idLyr = charIDToTypeID( "Lyr " );

        var idOrdn = charIDToTypeID( "Ordn" );

        var idTrgt = charIDToTypeID( "Trgt" );

        ref10.putEnumerated( idLyr, idOrdn, idTrgt );

    desc23.putReference( idnull, ref10 );

    var idFTcs = charIDToTypeID( "FTcs" );

    var idQCSt = charIDToTypeID( "QCSt" );

    var idQcsa = charIDToTypeID( "Qcsa" );

    desc23.putEnumerated( idFTcs, idQCSt, idQcsa );

    var idOfst = charIDToTypeID( "Ofst" );

        var desc24 = new ActionDescriptor();

        var idHrzn = charIDToTypeID( "Hrzn" );

        var idPxl = charIDToTypeID( "#Pxl" );

        desc24.putUnitDouble( idHrzn, idPxl, 0.000000 );

        var idVrtc = charIDToTypeID( "Vrtc" );

        var idPxl = charIDToTypeID( "#Pxl" );

        desc24.putUnitDouble( idVrtc, idPxl, 0.000000 );

    var idOfst = charIDToTypeID( "Ofst" );

    desc23.putObject( idOfst, idOfst, desc24 );

    var idWdth = charIDToTypeID( "Wdth" );

    var idPrc = charIDToTypeID( "#Prc" );

    desc23.putUnitDouble( idWdth, idPrc, -100.000000 );

    var idIntr = charIDToTypeID( "Intr" );

    var idIntp = charIDToTypeID( "Intp" );

    var idBcbc = charIDToTypeID( "Bcbc" );

    desc23.putEnumerated( idIntr, idIntp, idBcbc );

executeAction( idTrnf, desc23, DialogModes.NO );

// =======================================================

var idsetd = charIDToTypeID( "setd" );

    var desc25 = new ActionDescriptor();

    var idnull = charIDToTypeID( "null" );

        var ref11 = new ActionReference();

        var idChnl = charIDToTypeID( "Chnl" );

        var idfsel = charIDToTypeID( "fsel" );

        ref11.putProperty( idChnl, idfsel );

    desc25.putReference( idnull, ref11 );

    var idT = charIDToTypeID( "T   " );

    var idOrdn = charIDToTypeID( "Ordn" );

    var idNone = charIDToTypeID( "None" );

    desc25.putEnumerated( idT, idOrdn, idNone );

executeAction( idsetd, desc25, DialogModes.NO );

this for the vertical

// =======================================================

var idsetd = charIDToTypeID( "setd" );

    var desc26 = new ActionDescriptor();

    var idnull = charIDToTypeID( "null" );

        var ref12 = new ActionReference();

        var idChnl = charIDToTypeID( "Chnl" );

        var idfsel = charIDToTypeID( "fsel" );

        ref12.putProperty( idChnl, idfsel );

    desc26.putReference( idnull, ref12 );

    var idT = charIDToTypeID( "T   " );

    var idOrdn = charIDToTypeID( "Ordn" );

    var idAl = charIDToTypeID( "Al  " );

    desc26.putEnumerated( idT, idOrdn, idAl );

executeAction( idsetd, desc26, DialogModes.NO );

// =======================================================

var idTrnf = charIDToTypeID( "Trnf" );

    var desc27 = new ActionDescriptor();

    var idnull = charIDToTypeID( "null" );

        var ref13 = new ActionReference();

        var idLyr = charIDToTypeID( "Lyr " );

        var idOrdn = charIDToTypeID( "Ordn" );

        var idTrgt = charIDToTypeID( "Trgt" );

        ref13.putEnumerated( idLyr, idOrdn, idTrgt );

    desc27.putReference( idnull, ref13 );

    var idFTcs = charIDToTypeID( "FTcs" );

    var idQCSt = charIDToTypeID( "QCSt" );

    var idQcsa = charIDToTypeID( "Qcsa" );

    desc27.putEnumerated( idFTcs, idQCSt, idQcsa );

    var idOfst = charIDToTypeID( "Ofst" );

        var desc28 = new ActionDescriptor();

        var idHrzn = charIDToTypeID( "Hrzn" );

        var idPxl = charIDToTypeID( "#Pxl" );

        desc28.putUnitDouble( idHrzn, idPxl, 0.000000 );

        var idVrtc = charIDToTypeID( "Vrtc" );

        var idPxl = charIDToTypeID( "#Pxl" );

        desc28.putUnitDouble( idVrtc, idPxl, 0.000000 );

    var idOfst = charIDToTypeID( "Ofst" );

    desc27.putObject( idOfst, idOfst, desc28 );

    var idHght = charIDToTypeID( "Hght" );

    var idPrc = charIDToTypeID( "#Prc" );

    desc27.putUnitDouble( idHght, idPrc, -100.000000 );

    var idIntr = charIDToTypeID( "Intr" );

    var idIntp = charIDToTypeID( "Intp" );

    var idBcbc = charIDToTypeID( "Bcbc" );

    desc27.putEnumerated( idIntr, idIntp, idBcbc );

executeAction( idTrnf, desc27, DialogModes.NO );

// =======================================================

var idsetd = charIDToTypeID( "setd" );

    var desc29 = new ActionDescriptor();

    var idnull = charIDToTypeID( "null" );

        var ref14 = new ActionReference();

        var idChnl = charIDToTypeID( "Chnl" );

        var idfsel = charIDToTypeID( "fsel" );

        ref14.putProperty( idChnl, idfsel );

    desc29.putReference( idnull, ref14 );

    var idT = charIDToTypeID( "T   " );

    var idOrdn = charIDToTypeID( "Ordn" );

    var idNone = charIDToTypeID( "None" );

    desc29.putEnumerated( idT, idOrdn, idNone );

executeAction( idsetd, desc29, DialogModes.NO );