The two appear to be good friends and confidants. Sizemore views her as a moral authority.
When we first met Janis and Sizemore, Janis is trying unsuccessfully to teach Sizemore to use Flower Power, in return for a fee of 8 Rands. Since she is unable to teach him, Janis offers to waive her fee, but instead Sizemore insists on paying her 20 Rands, possibly because there is a good chance that he's going to die, and so won't need the money himself. Sizemore admits to Janis that he didn't really expect to be very good at the form of magic Janis was teaching. The fact that Janis had apparently never before tried to teach Sizemore Flower Power, and that he had originally agreed to pay her for his lesson, suggests that Sizemore and Janis were not particularly close friends before the events in Book 1. However, Sizemore's generosity may have impressed Janis, because when Sizemore returns to the Magic Kingdom linked to Wanda and Maggie, Janis offers to pay much more money to get him safely unlinked.