Sure, but the separate window can be on a different domain. Now you have a way to share cookies across multiple websites on different domains if all of them include an iframe to this external domain. And you can use in-browser messages (see window.postMessage()) to communicate between iframes and main window.
For how much he lost he must be buying avocado toasts with little diamonds in them, and then getting his teeth remade every day after they got destroyed by chewing the diamonds.