Since user input determines if the compare validator is enabled or not, then you need to use JavaScript/jQuery code, like this:
// This will enable the validator
ValidatorEnable(document.getElementById("RequiredFieldValidator5"), true);
// This will disable the validator
ValidatorEnable(document.getElementById("RequiredFieldValidator5"), false);
So you will need to run the above code in a change handler for the radio button list, like this:
$("#<%=RadioButtonList5.ClientID%> input").change(function() {
if($(this).val() == "Yes") {
// Enable or disable compare validator
}
if($(this).val() == "No") {
// Enable or disable compare validator
}
});