Depends on what he’s describing.
If he’s just saying that he has a spoken command come out of his Bluetooth speaker and that tells his echo what to do, sure, there are people doing that. That’s become a pretty common setup with a nestcam (which has two way audio). That’s no different than a person standing in the room and saying the command.
If he’s saying that he speaks into a device with Bluetooth and he has sound come out on the echo, again, that’s a standard feature – – unlike the Google home, the echo itself is a streaming Bluetooth speaker and you can play sound on it from another device.
You can also hold the echo remote in your hand and use it to give commands to echo. And that remote is paired to the echo by Bluetooth.
So there are lots of possibilities, just depends on exactly what he meant. 