It depends on where your event is being executed; within the context of the frame, or within the context of the parent.

Assuming you have everything on the same domain and don't have an issue with cross-site scripting restrictions, you just need to specify the proper context.

For example, if you have a function in your parent form called "fieldUpdate" and you try to call in from the iframe with "fieldUpdate();" you will get an error.

If you call it with the following, it will work (assuming there's no cross-site issues) because you're telling it to execute in the context of the parent.

parent.fieldUpdate();

If you are going the other way around and grabbing an element/value from within the frame, you have to specify the source as follows

$('#myFrame').contents().find('#myObject').val()

I don't know enough about how you have everything set up to give a specific solution, but I have a form that passes info back and form between the parent form and a page in an iframe so I know it can work.

If you're able to edit the javascript for the page in your iframe, I would suggest creating a function in the parent form that can receive a value and update the target field.

Then, in your iframe page call that function using parent.function() and pass in the parameters to let the parent form make the actual changes.

For example, your parent form would have the following

function updateField(field, value){
    $(field).val(value).change();
}

And your iframe page would have code similar to one of the following examples to trigger the change using the parent's function

// target the field with a custom class
parent.updateField('.myField input','test');

// target the field by ID
parent.updateField('#Field10','test');