How Can I Get A Refund Of Unused Philippine Airline Ticket?

My Mom and I traveled to Philippines for a visit from US last January but my Mom decided to stay in the Philippines for good. I returned back to US alone last February leaving her behind. I bought the 2 round trip tickets from PAL using their website and charged it to my credit card. How can I get a refund of her unused ticket from Philippines back to US? What info do I need to provide?