My girlfriend (British Citizen) will be flying to Canada from the UK on an eTA and requesting visitor status from the border official. She plans on possibly applying to extend her visitor status at a later date before the 6 (or 3) months is up, so she does not have an outgoing flight. The whole reason she's coming on a visitor status is in hopes to receive the invite from the IEC visa pool to apply for a working holiday visa and stay for 2 years on that. Will the border official turn her away/deny entry for not having an outgoing flight or will proof of funds for living expenses and later flights be enough? Also, is there a high chance she'll be turned away for saying she's coming to Canada and staying with me while waiting for an IEC Working Holiday invite to apply?
As mentioned in the comments, she doesn't need to have a return flight, but she needs to convince the border officer that she will leave the country again.
There are many ways to do that, and it could be very easy or very hard, nobody can predict that.
Put yourself in the officer's position - what would you find convincing? A return ticket is a strong argument; as is a job and/or husband or kids in here home country (as they are not easy to fake). On the other hand, a smile and a verbal promise - even if honestly meant - will be rather thin.
Better prepare for the tougher questions, and bring as many arguments as possible.
You can always buy a refundable one-way flight with a credit card, and cancel it after entering. But the officer might know that too.