She'd probably pay him by teaching him and by sponsoring him in the future? She also didn't took the portrait, so this kind of payment, by teaching, and sponsoring/investing on him in the future is more than enough of a payment if I were the artist.
Coming from someone who paints traditionally and digitally, this is my opinion
I don't know much about art or art history, but it feels like letting these renowned artists get a easy sponsorship could make their mental state deviate far enough to prevent them from making great works later on.
@Lunaion I was thinking the same thing. Art is often inspired and motivated by people's personal experiences and not just their ability. I have drawings and short stories that sprang from a particular thing that happened to me, so by blooming them early she might keep that inspiration from them.