Why do you say that?
It's up to CBSA to determine admissibility and with a visa (or eTA) the airline shouldn't have grounds for refusal because of a one-way ticket.
The traveller will have to convince CBSA that they are able to support themselves (or be supported) while in Canada without working and that they have significant ties to their home country and the funds available to purchase a return ticket.
That being said, it is a much easier with a return (two-way) ticket. Often, too, this can be much cheaper than two one-way ticket, even if you have to pay change fees.