Ray will contact you and let you know he's found a lead on Florian Cravic. You're meeting a guy called Talbot Daniels who will lead you to him.
Roman, My Cousin!
Niko will then call Roman and tell him that he needs his help. Grab a vehicle, or a cab and make your way to Roman's apartment. Pick him up and make your way to the GPS destination where you'll find Talbot. Talbot will eventually agree to show you where he lives. He'll physically give you directions as you're driving him around, so listen to what he tells you and follow the right roads.
Eventually you'll find the apartment. Watch the cutscene where Niko learns that the guy who betrayed his old army squad wasn't Florian, but a man called Darko Brevic.
Once outside, Niko tells Roman he needs to be alone.
- Florian's house is a few blocks East from middle park.