Quid pro quo (Latin for "something for something" [1]) indicates a more-or-less equal exchange or substitution of goods or services.
*It is my assumption that they have a "business arrangement". If she does not accept monies for her referrals, she probably excepts other items or services in return. It could be things such as SS pays for certain services for "their team" (*computers, phone, legal fees)
*&/or SS refering parents to Isabelle for coaching before &/or after the enter a program. If Sue refers her services each time a parent calls.. they are "offering" one more service to the family--- that generates an abundance of leads for IZ. The intention may be that Isabelle "coaches" the family while the child is away to prepare them for the homecoming. This could be true in reverse if a parent calls wanting help finding a program for their child .. she refers them to Sue (obviously).
*The team could be more intimate than we suspect. They could be in a relationship and possibly money is not an issue.