Instructions
Watch the cut scene revealing the identity of the mystery contact.
Go to the target location, which is The Viewpoint near Union Drive East.
Talk to Francis. You will receive a text message that includes a phone number.
Press "X" to call the number.
Walk around and look at the people on their phones. Listen for the blackmailer's voice both over the phone and in the park. If you play this part multiple times, the blackmailer won't be in the same location every time.
Shoot the blackmailer when he runs. He will drop evidence, so pick it up and hand it off to Francis before the police arrive.
Leave as quickly as you can. Try to lose the police that follow you. Your best bet is to speed away, but if you can't do that, make a lot of turns or hide in a safe location.