We need more info Birdman. Is the tank full or is he just putting in $5-10 worth of gas.
How far "down the street" is he driving before he parks?
How long is the car sitting before he comes back and starts it up?
Is there a wet spot on the ground behind or under the car when he gets back?
The problem could be anything from a bad gauge to a pin hole in the gas tank or fuel line.