I think your dad's mechanic is mistaken... although there may be a reason for it. On the G35 and other 3.5/3.7L engines, the gasket and rocker cover are all one piece. So in that case, the entire cover must be replaced.
Fortunately on your VK45DE, the gaskets are available separately. MSRP $30.28, available online at
http://www.orderinfinitiparts.com for $20.59 each. P/N 13270AR010 and 13270AR000. The covers are made of a composite (plastic) so in the event that there's an actual crack in the cover - you'd need to replace them too. MSRP $245.25 available for $166.77 online. Part numbers 13264AR001 and 13264AR011. Keep in mind that shops make money by parts markup plus labor charges... My HVAC technician marks up refrigerant about 40 times the cost... not sure what your dad's mechanic does. The typical model is that they buy the parts for roughly what I'm showing as an "online" price from the dealership (unless they're using aftermarket parts) - and then an hourly labor rate based on "book time" - which is the time the factory establishes as the time it should take to complete a job. The technician will try like hell to beat "book time" to turn his 40-50 hours of real work time into 50-60 hours of paid time.
When you're looking at these things - Infiniti's parts site can be very helpful.
http://www.infinitiusa.com/parts - once you find the part number you can google it for an online price.