I think that ostracism isn’t just a case of choosing who you mix with, it is publicly telling others to also ostracise the subject and that, if they associate with him, they too will be ostracised.
In most avenues, it is not considered acceptable behaviour. In the workplace ostracism is considered bullying and can be used in constructive dismissal claims.
There have been a few articles in The Times about this case, one saying that he had a meeting with his friends when they told him they ‘needed space’ and would ‘check up on him in a couple of weeks’. The second was a first person account by an Oxford student whose friend had been accused of racism. Others set up an Instagram group to tell the year group not to contact him and, if they did, they too would be ostracised.
So, I think there are two issues. It isn’t the girl’s fault that she chose to share details with friends, but their reaction that seems the issue. I also think that when the coroner mentioned ‘cancel culture’ he clearly had more facts, and these do seem substantiated by a broader issue in student relationships and social media groups.