Hi,
I've already had the same problem and I've solved it with this js:
function ValidateComments(source, arguments)
{
var comments = NWF$(".nf-comments").find(":input").val();
if (arguments.Value == "2" && comments == "") {
arguments.IsValid = false;
}
}
you have to put the tag nf-comments as css class of the comment field and change the value 2 to the one related to the outcome of your task.
Because that number isn't always the order of the outcome, you have to find it (I've inserted an alert(arguments.Value) on the first run of the task to discover it).
Hope this help.
Giacomo