Hidden City ticketing is specifically prohibited in UA's conditions of carriage. Passengers who book through UA's website agree that they will not use Hidden City ticketing.
Just about every other airline on the planet (that have through fares via connecting points) specifically prohibit the practice. Easyjet, Ryanair et al, don't prohibit it but who cares, you can't do it on a point to point airline.
The young man in question is counselling passengers to breach their contract terms with United airlines. That is what this case is about, not the validity of the prohibition.
If caught the passenger will be refused transport, again pursuent to the contract terms they have agreed.
The courts and consumer agencies have reviewed these rules (ad nausium) and have always (finally) upheld the airline's position that such terms are fair and reasonable.
The EU is "addressing" the issue, in their draft rewrite of Regulation 261, by requiring airlines to accept hidden city passengers but allows the airline to charge the fare that applies to the journey the passenger actually (intends to) takes. In other words they agree that lower fares cannot be secured from Hidden City ticketing.