What is the ruling on a foreign driver seeing the face of a mature girl?

A woman must observe hijab in front of a driver and a servant if they are not her mahrams. It is not permissible for her to uncover herself in front of them, nor to be alone with either of them. This is based on the Prophet's (peace and blessings be upon him) saying: "No man is alone with a woman but Satan is the third of them," and on the general evidence for the obligation of hijab and the prohibition of adornment and uncovering for non-mahrams.