This is usually caused by an outdated Plex app. If the tv is an older model, it might not be compatible with current cryptographic certificates. Important information about Plex for smart TVs after September 30, 2021
As a test, ask him to use a computer and go to https://app.plex.tv
If he can use this successfully, you know at least that the remote access is working.
(It won’t help him though.)
Perhaps he already has an external plug-in device for which a Plex client is available. Or you could gift him one. Category : Player Apps and Platforms | Plex Support
