Babcock isn't an OC - so his mentorship probably had a great deal to do with Gattis' professional relationships with other people; navigating the college football coaching world, etc etc. That means that Babcock was privy to information regarding Gattis and others that he wouldn't have been if he was not a mentor. If he hires him, he immediately becomes his boss, not a mentor, where Babock's first concern has to be his role as AD of VT and not Gattis' professional development. Could Babcock have manipulated the information he learned as a mentor to Gattis for Babcock's benefit? Did his advice steer Gattis towards leaving his current role? It's unethical in the same way that a therapist advising someone about their personal relationships should not enter into a personal relationship with the patient.

EDIT: I read the article linked above and absolutely it would be unethical. Babock is mentoring Gattis on becoming ready to be a P5 coach and how to go about it from interviewing for the job, presumably including contract negotiation. Gattis is going to get a job somewhere, it would immediately raise concerns with me that Babcock could take advantage of him through the entire process were he to become a candidate for the VT job.
[Post edited by IronBlood at 11/21/2021 6:39PM]