There are many Orange stores around, but you probably need go to Orange store, not an authorized dealer. There are many. She'll know where to find one. You buy the SIM cards at the Orange store, they activate it for like RD$350 or something, and give her the phone number. I believe they give you a free prepaid card as well at that time. The Orange prepaid cards are sold everywhere, in denominations of RD$50, RD$100, RD$200, & RD$300.
Also, I believe Verizon has stores around as well.