although not the exact same issue, i too faced a similar drama some years ago at the hands of the clueless staff on duty at the time.
in a nutshell i was returning to work in NZ (i work on a ship) and as such i had a one-way ticket from YYZ-AKL with supporting documentation to show not only was i in transit, but also that i was not remaining in NZ for any amount of time greater than that required to get from airport to ship.
on arrival at the counter, and after much back and forth i was told that i required a visa (Canadians do not need a visa to enter NZ) or i needed to have a return ticket despite me proving that i was not staying in the country.

the long and short of it all was my options were to either go home and call my company the following morning (it was a Sunday) and tell them i wasn't returning to work because i was denied boarding, or i had to purchase a fully refundable one-way ticket from AKL-YYZ to 'prove' to AC that i was leaving NZ and as such satisfied some absurd ruling despite already proving i was leaving the country on arrival. thankfully i had a credit card on hand with enough available funds to purchase the fully refundable one-way ticket which i then cancelled shortly after arrival, but its the headache and stress involved which could have been avoided had the staff present not had their heads up their backsides...and i'm a native English speaker so i can just imagine how frustrating it could have been for the OP's aunt!!