First of all I would try to make him comfortable since a victim of cyber crime gets way too much depressed and in despair.
Then perhaps, I will try to find out what the matter actually was.I mean on what basis he has been harassed online.
If the matter turns out to be 'not-that-serious' then I will try to find what remedial actions can be taken.But if it turns out to be serious,then for sure I'll help him in seeking justice,even if that means taking the legal help which is POLICE.