Mom:
By "hacking" do you mean getting into his account(s) and changing his password? E-mailing other people, what? To "hack" means to get into someone else's account and change passwords, send e-mails, etc.
Do you have any proof that she is doing this? Any notifications with an IP address that you can take to the police to show as proof.
If you know her parents - since they were dating for a year - I would HOPE you know her parents....talk with them. Let them know what you feel she is doing. Let them know you plan on making a report to the police if she doesn't stop.
Then I would have him make his passwords stronger - alpha numeric - with a character and capital letter, for example:
4_Me2Know
3Kids4Me&You
NOT his date of birth
NOT his name
NOT his dog's name
You get it - right? Things that she won't know about him.
Change all of his security questions as well.
Ensure the firewalls on your computers (I assume you have a network) are up and working. You can make a spreadsheet for the passwords - you put hints ONLY YOU and HE would know - not the actual passwords so if the network is compromised, she can't get into everything.
You and he need to report this to the police. Call the non-emergency number and file a report.
Personally? I would have him close all of his accounts - facebook, twitter, e-mail, etc and not re-open them until she is stopped. I know. I know. I know. He should not have to stop living his life because of her. I get it.
However, does he NEED them? No. He does not. He WANTS them.
If he is going to have an account? He needs to make sure ALL of his friends are people he knows personally. Let them know he has a "stalker" and she is trying to get into his life.
Good luck. If you have proof - print it out, take pictures, etc. talk to her parents and then the police. Make sure he changes ALL of his passwords.