It is never explained how Foster somehow exited the vehicle in the storm after being stuck inside because of the command lockout on the airlock door.
As mentioned in "The Martian" (2015) :
Due to Mars' low atmospheric pressure, the effective wind pressure in martian wind storms is quite low. The dust on Mars at the surface tends to be very fine, with grain sizes comparable to smoke particles. There are dust storms on Mars, but they look and behave like puffs of smoke instead of raging wind storms.
As already mentioned, the atmospheric pressure on Mars is low, and even after terraforming (in this movie) people do need oxygen masks. Thus a storm would never be able to enter a pressurized dome. If by some accident the dome broke, the air would blow OUT of the dome in opposition to dust and sand pushing into it.
The helmets used on Mars are very badly designed. The visor area is only a small square at the front with no peripheral view, unlike those of real astronauts/cosmonauts or those in every other science fiction movie.