There is not going to be a specific rule that says a team can't allow a representative from another team on their sidelines because that is absolutely ridiculous and nobody in 100 years would expect that to happen. I bet there also is no rule that says representatives from other teams can't dress in disguise and wear a different team's official gear.But again there’s nothing specifically that prohibits someone from being on the sideline if they work for another staff simply because they work for another staff.
From 2020 but assuming no big changes.
Section 4 covers the Sportsmanship Rules.
Live scouting is covered on page 38.
Football starts on page 77.
2021Handbook (PDF) - Mid-American Conference
getsomemaction.com
Not seeing anything about visitors outside of the number of field passes (60). From CMUs perspective you'd need to prove they knew he was there to scout other schools it appears to. Obviously a big problem from the UM perspective.
If there is anything it would be covered under sportsmanship or something like that. I don't care enough so I'm not going to look, I was just commenting in hopes that you'd stop asking the same question over and over and shitting on this thread.