Have Joey talk to Peter and see what you can learn.
Hint 2
Not much, as it happens. Well, we did find out something: Peter's spirit hasn't yet been destroyed. Let's go back to Madeline and Michael and tell them that.
Hint 3
When you talk to Michael about Peter the second time, Michael tells you that Peter used to give personal training sessions. That will give Joey more to say to Peter --
Hint 4
-- who will now give you his phone number. Take it to the police to trace it, then go to Peter's apartment to see what you can learn there.