From an employment law perspective, Red Bull's case is likely to be that the Grievance Policy and Procedure was followed, it was found to be not upheld, and there has now been a breach of privacy and confidentiality with the potential/alleged leaks to other teams and the papers, leading to a potential breakdown in confidence in the employer/employee relationship.
On the allegations against Horner themselves, whether you think there's much in the content of the messages that he sent or not, the fact remains that he is in an extremely senior and powerful position at Red Bull and therefore his behaviour towards a junior member of staff is unacceptable. The optics aren't fantastic here from Red Bull.
