Hello Khaikhok, yes they must need a return ticket if they are coming in visit visa otherwise they may not get entry at the port.

Otherwise, It may show the intention of no return. I am not sure if the immigration officer ask to show the return ticket.

It's better to buy a round ticket I suggest. You can always change the returning flight ticket once they entered, or ask for a refund of the return ticket price if don't want to back so why do you want to be part of the problem yourself buying only a one-way ticket. This is only my suggestion, it's up to you.

My mom came to visit us on one way ticket.