Hi, I have a Tablo 4 tuner, but it's always been hardwired to my network, so I don't have any direct experience with configuring a Tablo to work via wifi, let alone changing the password after it's connected. However, I've had a similar sort of issue other wifi connected devices like Chromecast Audios and changing wifi settings so maybe I can be of assistance.
First, I hunted down your original post from the end of May. Based on that, your Dad is using an Apple TV and a phone (iPhone?) to access the Tablo, correct? Have you followed the directions in the guides that TabloSupport linked to in that thread, especially this one?
In particular, can you connect a phone or other device to the Tablo's own wifi signal, which it should be sending out if it can't access a network?
Also, I see theuser86 has asked the question I was going to next. Can you connect via Ethernet rather than wifi temporarily, and does that resolve the issue? If so, can you then go through the link above about resetting the network password? (Perhaps something in the wifi settings have become "stuck" and switching to wired and switching back to wifi will allow them to be reset.)