At the top of the home page there should be an option to change the input to the cable box. They can still use the TV without an Android phone. Just download the Android TV app to their phone.

I never touch my phone using my Philips. You connect it to your Google account to get services. This is unnecessary as it still works as a 'dumb' tv. I use mine with my HTPC this way. The main benefit is I use my providers streaming service and disconnected my box. Less heat.

Smart TV, Android or anything else does not matter in your case. The TV is only a display and you connect your cable box via HDMI as an external input. Since most TVs at least newer than 10 years old support DVB-C. You could probably buy a CA module and skip the cable box completely. Resulting in one less remote control. At least in Europe maybe not in the US.