The IOS notification is an option, but it does not solve the issue when the user is sitting at their PC. In the last round of WTH (Why the Heck can’t we have …) I submitted for a true popup card. One that would display on the PC, mobile app or any other device that supports HA. See here: WTH - Can't we have a true PopUp Card?
Seems like this would be the optimal answer for this user and many others.