How much should I pay for a portrait done by a friend?

I close co-worker of mine is an excellent painter and paints in her free time outside of the office. She is currently doing a weekly studio session where she can paint whatever she likes, as sort of a treat for herself each week. She is a busy mom of two and doesn’t get to spend as much time as she’d like on her art.

She recently painted a portrait of my new puppy in her class. The canvas is approx. 14×18 inches and took her three sessions to complete.

Of course she has not asked for anything for the painting, but I would like to compensate her for it.

Does anyone have any recommendation on what the going rate for a portrait of this nature would be? Or perhaps a suggestion of how I could repay her?

Thanks!