Martina,
Communication between iFrames is not really an esri GeoNet kind of question.
Normally you have a global var that references the opener. Something like this:
document.winobj = window.open("someurl", "_blank", features);
Then that html file has public functions or methods that you call using document.winobj
Then in the html file I opened, it can get access back to the widget using something like this:
window.opener.document.myWidget.setLatLonHandler(pt.lat(), pt.lng());
The document.myWidget was set inside the widgets postCreate method:
document.myWidget = this;