He received what he paid for: wifi services for that flight.

I still don't see fraud here.

He paid for a WiFi connection between a phone and the internet. He received a connection between a laptop and the internet.

He paid for a WiFi connection between a device and the internet. He received a connection between a device and the internet.

If GoGo can't tell the difference between a laptop and a mobile phone, that's their problem. And no, the UA string doesn't guarantee that and there is no law that I've heard of that prevents users from altering their UA string (or anything for that matter). They showed him a price for a service on his device, and he bought that service.

Ha, good luck trying to argue that. The Gogo website makes it clear that there are separate services for phones and laptops. Their website automatically detects what kind of device you have, if they make a mistake then it's their problem if they don't offer you the means to correct it. But if you deliberately circumvent their system to save money, then you're committing a fraud.

I'd urge you to learn a little more about the law if you think that a UA string specific law is needed, or even a computer-specific law. Intent and personal gain are more an enough.