However allegedly there is a smell of lpg coming in through the air vents
on
the dash.

If they had just been driving the car on gas, then stopped the engine, then
there will be a smell of gas, as the gas in the inlet manifold will escape
via the engine air intake. This is perfectly normal and the only way to
prevent it is to stop the engine by shutting off the gas, rather than by
switching off the ignition.

It is almost impossible that the smell of gas is entering the vent while
driving, as any gas that leaked would be immediately dispersed by the
airflow around the car.