I don’t understand this, since that’s not how wireless radio transmission works. If you get two signals at once, they will more or less destroy each other. This is why having a plethora of 2.4GHz devices in the home is bad for interference. The only way this could happen is if his camera isn’t transmitting.
Quite possible. Many people wire those cameras to the reverse lights 12v circuit, so they don't transmit continually - just when you're backing up.
I would assume that his camera is only transmitting when his vehicle is in reverse (that's the way mine works). When he's driving forward he shouldn't be getting interference from his own camera.
What if he tapped into the backup lights to get the power for the camera? The camera would only transmit when the truck was in reverse. The receiver/display, OTOH, is taking it's power from the cigarette lighter. It is free to receive any and all incoming signals.