I have a client asking about the ability to view his other houses cameras on a TV in his main residence. Is this possible from a internet connected TV? I don't think so, but maybe I can download the app if it's an Android TV??
Have any of you done this, somehow?

only on an app

Don't overthink this. Get a PC with an HDMI output and connect it to the TV.

Use standard cameras/DVR that let you remotely view cameras via a web browser or PC software.

We have done this a few times (not using the TV Apps), either using the Axis decoder P7701 which is OK and enables you to view 1 image at a time and you scroll through the cameras one-at-a-time. Or more successfully using Brightsign media players, this can display several screens and also weather, storm warnings, local news at each house location etc.

Dupe...

Digital Watchdog now has an app for AppleTV that allows viewing cameras, but I haven't seen anything about whether it only works for on-site cameras or if it allows entering the remote address.

This is great. [Link: digital-watchdog.com]

Says....

"DW Site Viewer™ app for Apple TV lets you quickly enter and save the information for one or multiple surveillance sites and then choose to monitor the cameras from one of those sites with the click of a button."

I think that means you can do other houses. Also if you upgrade to the pro version/monthly subscription you can review previously recorded video from the app. Otherwise its just live viewing.

Some NVRs allow you to configure remote players as feeds.
Bandwidth and overall monthly data usage may become an issue thought.

Yes, but you will have to write your own app for the TV.

Samsung has a public SDK.

Or you could use an encoder at the site and a decoder at the TV.

Or a PC like others have said