All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If your husband did what you mentioned in the question, and he was alone with this woman in the car, then he has committed a reprehensible act. Many contemporary scholars issued their Fatwa that the presence of a woman alone with a non-Mahram man in the car is regarded as forbidden seclusion. The sin is graver when they travel together because he would be assisting her in committing a sin. It is impermissible for a woman to travel unescorted by a Mahram, as authentically established in the Sunnah.
Thus, it becomes evident that your husband clearly did wrong by doing so and has fallen into sin.
This act is prohibited, whether it is done by the driver or your husband, so there is no point to use the situation of the driver as an argument to justify what he did.
It is incumbent on your husband to repent to Allah, The Exalted, sincerely. The conditions of sincere repentance are highlighted in Fatwa 86527.
You have the right to feel hurt and angry about his act; protective jealousy over one’s Mahrams is Islamically required. It was narrated on the authority of Abu Hurayrah that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Allah has protective jealousy, and the believer also has protective jealousy. The protective jealousy of Allah is provoked when the believer commits what Allah has forbidden." [Muslim]
It is obligatory to forbid this conduct and advise him in this regard, but it should be done gently and at an appropriate time.
That a man follows the social media accounts of his female colleagues and friends may be a means leading to Fitnah (temptations) and to committing what Allah prohibited. The elimination of means that lead to committing sin is obligatory.
Allah Knows best.