Thanks for posting. To confirm, is the Chromecast with Google TV already set up? If yes, are you trying to cast a video content using your partner's iPhone? Are you getting a message that there's no cast destination found?
If your partner's iOS device is running on iOS 14 and above, please enable the Local Network Access. See the steps below:
Just checking in to make sure that you've seen my response. Please let me know if you have other questions or concerns as I will be locking this in 24 hours if I won't hear back from you again. Feel free to start a new thread and we'll be happy to help.