Upvote:1
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.