[Last para:] If you do decide to go ahead with a return, keep in mind that the return leg doesn't need to be the same as the outbound. You might be able to book ZRH-AUS on the outbound, and a cheaper/shorter flight such as JFK-FRA for the return which may give a cheaper overall flight. Given that you're not going to fly it anyway, it doesn't really matter where you're "flying"! Try a few options and see what you can come up with.
For brevity, call the return destination on the return ticket as the spurious destination. The motivation for this question is my friend's need to buy a one-way flight from Hong Kong to Toronto. If a roundtrip ticket is cheaper, then how do we compare and determine which city on the return journey (that my friend won't really fly on) will produce the cheapest roundticket fare? Are there any websites for this? For example, we simply don't know if Hong Kong --> Toronto -> Hong Kong is the cheapest, or Hong Kong --> Toronto -> X, where X is some other city besides Hong Kong.