It does not appear that she and Ronaiah were actually close or even friends. They had a social media connection, but both stated that they didn't know each other well. That lends credibility to the idea that she was doing this as a favor for a sick kid, because there really is no other reason she would agree to it if she didn't know Ronaiah. It's not like he had money that he could pay her to do it. Here, you had a "Christian musician" reaching out and telling this young woman that a sick child would love a picture of her, and can she hold up this sign in a selfie to show that it was "just for her."
If they were lying about how well they knew each other, I don't that would've gotten past the filmmakers. They dug out everything.