Badman tells Niko to go to Wallkill St in South Bohan. Stop in the marker and watch the cutscene. Follow the dealer on foot, being carefull not to spook him. If the dealer does get frightened, he will start running. This will not fail the mission, but it is more difficult to keep up. Roman will call during the mission, and the phone may scare the dealer, so hang up the phone as soon as it rings. Follow the dealer into the apartment building, but don't kill him.