My step brother got a new galaxy tab 2 from ATT yesterday and I told him I can set it up so we can have multiple user accounts thus sharing it without having our privacy invaded by each other. little did I know this tablet is running android 4.0.4 and not the required 4.2 that has multiple user accounts built in. My question is this. If i install a custom rom onto the divice will that completely destroy the link to ATT that he has? mobile networking through the towers and the likes? Also will this completely void his warranty / insurance?
No, he will still have at&t's service. And it would void the warrenty, but if you can return the device to stock, warrenty should still be there.
Perfect. How would I go about reverting to ATT stock rom? Do i do a device reset and it does it automatically? I dont have a problem really figuring out how to do it its more of a (is it frustratingly glitchy or risky).
Although I have only Nexus devices, the process should be the same. The same way you flashed a custom ROM, just flash the stock ROM. Or make a Nandroid backup of your stock ROM after rooting and installing a 3rd-party recovery but before flashing a custom ROM. That way all you have to do is just restore the backup. Keep that backup somewhere safe on your PC, and make sure to factory reset, wipe cache, and wipe dalvik cache, before restoring or flashing the backup to avoid any problems.
This is just an oversimplification. Read up on some guides for your specific device.
I'm not sure if the process is the same with Samsung tablets as Samsung phones, but for phones you just flash a stock RUU via Odin, and everything will be stock, including the recovery.
Just dig around on the XDA forums. There should be a thread/FAQ/wiki/guide on this for your device.