If the harassment is happening on bungie.net, please report it to the Ninjas to take a look at. If it's a message such as a PM, you can directly report that message from the message options; otherwise, you can speak to any Ninja about it by contacting them:
If it's happening on other platforms outside of bungie.net, unfortunately there's nothing Bungie or their moderators can do. You would want to reach out to that platform support to report them.
I also suggest you guys change your account credentials and enable 2FA for extra security if you haven't already.