Will she only require a single entry or double entry visa?

My girlfriend is Cambodian living in Phnom Penh. We will travel to Hong Kong & Macau for 5 days. A travel agent has quouted $35 for handling the application for a 4 day delivery.
Also is a return flight ticket needed to apply? We have not booked flights as wanted to ensure we had a valid one before spending the money as the travel agent has said there maybe a quota of applications that the embassy accepts and that recently apllications were denied due to more than 50 cambodian applications. I personally find this difficult to understand why this should be the case and do not necessarily believe.