Is it permissible to speak with a young man via social media with the aim of deterring him from toying with girls' feelings, especially if the account is shared and this is not considered a private meeting (khalwa)?

The girl must sincerely repent for what happened and commit to obeying Allah the Almighty. A woman's conversation with a non-mahram (definition) man is not permissible except when necessary, with adherence to Shariah guidelines and ensuring safety from temptation. If it is proven that the young man is toying with the feelings of girls and spreading corruption, he must be advised. If he does not repent, he can be threatened with reporting him to the responsible authorities.