For sure a messy rule. The undue influence question is a GREAT one because NOBODY is going to transfer their kid (for athletic reasons) without first determining where is the right fit for their athlete. Which, as a result, puts the coaches, AD's, etc. at risk of becoming a party to or active participant in undue influence. The IHSAA by-laws define undue influence as... "the act of encouraging or inducing a prospective student to attend a school for athletic purposes."
So for the sake of this conversation, the scenario @slicer28 lays out could very well fit the definition of undue influence. Parent calls and asks a coach or AD about moving their kid to their school, asks questions about the athletic department/ specific sport program they want their kid to join. Coach or AD answers ANY of those questions and boom, they just recruited, OR as the IHSAA puts it they just performed "the act of encouraging or inducing a prospective student to attend a school for athletic purposes." Even though they didn't ACTUALLY do anything wrong by answering mom/ dad's questions they can be said to have encouraged a prospective student to attend their school for athletic purposes based on how they answer those questions. As you and I said slicer, MESSY!