Hey I have a TCL TV with the latest Google TV software installed. I am able to cast to it no problem from my phone, but whenever my wife tries (the TV is signed in to my account) it just loads indefinitely. The TV shows up as an option for her but once selecting it it never connects and we don't get an error message either.
Are you both connected to the same WiFi?
If your router has both 2.4GHz and 5GHz networks, I have seen problems sometimes if the Chromecast is on the 2.4GHz and the phone is on the 5GHz. (or make sure she is not connected to a guest network)
As far as I know Chromecast doesn't have anything to do with accounts, so there is likely another issue. Could just be something weird with her phone. Have you tried it from another device or desktop browser?
Try it through a proxybus.