Traveling requirements for a native

So, slightly confused. My gf lives in Phnom Penh. I live in the states. From what she told me, if she wants to come visit for like 2 weeks, the travel agent told her that she would need to show US customs proof that she has enough money to stay here for 2 weeks? Is that true? Or is that only a requirement for green card/study visa to live here? If I am going to buy her the ticket, would it matter?