I have tried replicating the code that you have suggested based on the code generated by Form Builder.
I have ended up with
<?php echo((isset($_GET["EmploymentStatus"]) && $_GET['EmploymentStatus'] = "Employed")?ValidatedField("ApplicationForm","EmployersName"):"")); ?>
However this produces a Syntax error in dreamweaver.
I wasnt sure what I would use for the validation part, you used a telephone number example, as this is just a required field
Code for the Radio Button
<input type="radio" name="EmploymentStatus" id="EmploymentStatus__1" value="Employed" class="formRadioField_Standard" <?php if (!(strcmp((isset($_GET["invalid"])?ValidatedField("ApplicationForm","EmploymentStatus"):""),"Employed"))) {echo "checked=\"checked\"";} ?> tabindex="19">
Code for the Form Field that only requires validation if the above button is checked
<input id="EmployersName" name="EmployersName" type="text" value="<?php echo((isset($_GET["invalid"])?ValidatedField("ApplicationForm","EmployersName"):"")); ?>" class="formTextfield_Large" tabindex="24" onBlur="hideServerError('EmployersName_ServerError');">
Also I am unsure as to why everything is $_GET when the form was created as $_POST and also submits using $_POST