Fresh air duct and intake are two entirely different things. One provides air circulation to the passenger compartment, the other provides air to the combustion chambers. If it is coming in the fresh air duct I would suspect a leak at the top of the engine. Inspect the fuel rails, fuel line connectors to the fuel rails, injectors, fuel pressure regulator, etc. Use a flashlight, not a barbeque lighter, to see what is going on. If it is coming from the intake, check the fuel pressure regulator.
If your dad recently replaced the fuel lines, my main suspect is the connections from the fuel lines to the fuel rails.
Any signs of gas dripping on the road when parked for a while?
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)