Noor Mateen should be arrested for being an accomplice to the shooter. She drove him to the nightclub to help him scope it out. She knew when he was going to do it and didn't notify the police to save all those people's lives. She could've prevented this from happening.
Noor Mateen should not be arrested just because of her affiliation to Omar. Yes, while she may have drove to the club doesn't mean that she had wanted him to do this. Omar may have threatened her or her family if she did not do this. Omar might have threatened to kill her if she went to the police so it doesn't justify that she should be arrested.