Topic: Earning income from a foreign country while waiting for a green card (K-1)

I am aware that K-1 applicants waiting for a green card are forbidden for working in the US before they have their green card or a work permit. What I'm wondering is whether my fiancé, who is an author in her home country, can still collect income from book sales there while awaiting her green card. Each month she sells e-books and income is automatically deposited in her bank account (in Thailand). What are the rules regarding this? She can't stop people from buying her books once she arrives in the US, and therefore, would still be earning income. Any help with this matter would be greatly appreciated!