What is the ruling on a husband going out with his friend and his friend's unveiled daughter under the pretext of saving petrol costs, and is the wife justified in being concerned about temptation (fitna)?

It is not permissible for your husband to go out with his friend and his unveiled daughter, because his going out with them entails seeing her and her adornment, which is forbidden by the consensus of the scholars, due to the Almighty’s saying: ﴿Tell the believing men to reduce [some] of their vision and guard their private parts. That is purer for them. Indeed, Allah is Acquainted with what they do.﴾ His going out with them is considered an affirmation of evil, which is impermissible. He must denounce it in his heart by shunning those who commit sins and avoiding their company.