Impossibility is a defense to a breach of contract. And let’s not be silly; United’s not going to go after you for unintentional breach due to illness. Come on, use some common sense.
Are you arguing that buying a return ticket even though you need a (much more expensive) one way ticket and then not using the return segment is fraud?
Apart from proving that you intentionally bought a return flight best of luck defining this as fraud in front of a judge.
I booked two flights to Paris for my girlfriend and me. Before we left she found out that she has to return a day ealier. Instead of rebooking the flight (which would have been insanely expensive) she chose to return by train.
Arriving at the airport for the return segment I proceeded to the gate and informed the airline employee that my girlfriend will not make the flight and that I would like to check her out.
Instead of busting me and denying boarding (which would be the logical conclusion of your asessment) she thanked me profusely and I went on my merry ways (waiting for and eventually boarding the flight).
Now, you can certainly argue that we didn't know about her earlier return when booking the ticket. But we sure knew about it when boarding for the first segment. Nevertheless and despite the tickets running under the same record locator I did not have any problems boarding the return flight nor was I approached by the airline later.
Of course it could be that European airlines are generally not as hard nosed as United (they generally also don't rough up their passengers, but I digress). But I think it's more to the point that it's impossible to prove that I never intended to take the return flight. It certainly would be more obvious for hidden city / nested bookings strategies that I intended to save money (I would be very reluctant to call that fraud).
Moreover, fraud is a criminal concept. Violating the contract of carriage is certainly not.
Apart from proving that you intentionally bought a return flight best of luck defining this as fraud in front of a judge.