This one is will then fall under civilian case, police is may
not take the case. So you will have to make a magistrate
complain.
When you go to the magistrate, you will need to submit some
evidence first. 
The evidence is the harrasement emails. How to
get the harrasement email leh? You can go to the service provider,
telling them about your friend's current situation. Depend whether
the service provider is willing to go extra miles to help you dig
out the information from their system or not. Usually a laywer's
letter of demand is quite effective.
Once you get the evidence from the email service provider
already, then you can submit to the magistrate court. If you want
to make this a court case and make claims, you need to prove that
your friend suffered lost. A medical report will be needed to make
claim from this case as you mentioned your friend suffered mental
breakdown.
The process is super long without a lawyer's assistance. May
drag for years at least.
The money involved in hiring lawyer and amount of time and
resources spent may be high.
