Fraudulent use of hotmail account to receive unauthorized forwarded email from a hacked yahoo email account

My 85-year-old father's yahoo account was hacked and his email was forwarded without his knowledge or consent to a hotmail account that was created to mimic his yahoo email address - same front end, different backend (x...@yahoo.com and x...@hotmail.com).  The email is no longer being forwarded, but this person is still using the fraudulent hotmail account.  He would like to get the fraudulent email account closed. Microsoft live chat was unhelpful. What can he do?