Here's a better source (it has facts). https://law.stanford.edu/press/student-booted-in-bible-flap-suing-school-district/
(more facts) https://www.sandiegouniontribune.com/sdut-student-suspended-bible-seized-was-christian-disru-2011mar31-story.html
Turns out that in 2010, Kenneth Dominguez was suspended for being disruptive because he wouldn't leave the other students alone. He was not suspended for carrying a bible to school.
So, if Kenneth Dominguez had done exactly what your friend had done, he would have been fine. He was preachy about it. And he didn't leave people alone.
And if you're like "well, how disruptive could he be?!?" The school is small. Gateway East, in downtown El Cajon, has 34 students, two teachers and two classrooms.
He wasn't in some big highschool. He was in a small school, and he wouldn't leave the other students alone about his faith.
Yeah, I can see why he got suspended.