Anyone have a way of creating a pop-up window on a user workstation after a user action is taken

We currently send an email to a user if they have not updated accounting information correctly with an invoice approval flow. I'd like to have the flow check the accounting information and then push a pop-up window to the user's workstation rather than send an email. Does any one have any ideas on how to do this? I've been looking at the Custom Activities Node as a starting point.

Thanks in advance!