Hmmm well what i think you can do. Is if you had previous access to the account is to call blizz up and tell them that your account got hacked or make up a story about someone you gave all the account info to. If you're good enough and they buy it (they believe things most of the time), they'll temporarily ban the account and an email will be sent. Make sure that you tell them that he changed the info so the email doesn't go to his email.
What blizz will do in most cases is ask you to fax a photo I.D along with a filled out form of account info. Hope everything works out =(.
Same thing happened to me but I don't think it was the guy's fault. I trust him and everything, but minutes after, i went to the grocery store and decided to call blizz to change contact info. They ask me the usual questions, the sq/a, the name, email, etc. They asked for CD key...didn't have, last numbers of credit card, didn't have, and some characters on the account. So I [censored] up on that but they proceed to temporarily ban my account until I send photo I.D and fill out an app. I talked to original owner, he says he's sorry and working on getting it faxed, hopefully everything will work out. I'll post here if i get screwed.