I wouldn't have given up on a true freshman and his lack of defense.
What should Miles have done differently?
Obviously I can't speak for what went on in practice or in meetings. I can only speak on what I witnessed on the court. Some players are offensive minded. Some are defensive. Some can do both.
Horne was obviously an offensive minded player. And a threat to score when he was on the court. But Miles kept pulling him and reducing minutes for "lack of defense." No one on the team played consistent defense every game.
We had stretches of games where we wouldn't score for 5,6,7 minutes or more. And Horne, who was pretty good at stretching the defense, was left on the bench.
So What Miles could have done, was continue to use Horne offensively while still working with him to play better defense. Like I said, he was just a freshman. And as we have just seen with Webster, maybe giving up on someone too soon isn't the answer.