First of all he probably used 'SQL injection' to hammer the PM accounts thats what I would do.
Also to get back at his BB system google for PHP BB exploits in google.
Then what I would recommend highly is signing up 20 users accounts to his board at
http://board.*************.com/
Next you basically copy his page to 'post a new topic' onto your local C: drive. Adjust the variable so you are posting from your C drive or a php webpage you've built directly to his server.
Use a nested for loop to create a flood attack and post 20,000 new topics make sure your subject has a varible with an arrary[i] so new subjects changes on every post.
Flood his board with ****. After a while you'll corrupt his mysql database and it might toast his entire site. Thats what I would do. Only take about 1-2 hours to set up and a couple hours of your computer on auto pilot to break his ****.
If the BB has flood controls which it probably does, just adjust your for loop accordingly.
The FBI won't do **** unless its over 5k in damages, since he is overseas just about anything goes.