Does sin befall the driver for obeying his relative’s command to stop the car, with the intention of seizing a phone found by a vendor, and is he sinful if he is unable to retrieve the phone?

The ruling on the aforementioned mobile phone is the ruling on a lost item (Luqata), and its finder was not permitted to hand it over to someone untrustworthy. It is not permissible to dispose of a lost item before announcing it and a year has passed since its finding. There is no sin upon your brother for what happened, and his responsibility is to advise and guide. If he is able to recover the mobile phone and return it to its owner, he is obliged to do so; otherwise, there is no blame upon him. He did well by abandoning the company of the bad companion, but he should not abandon him completely if that will not deter him from his corruption; rather, he should advise him and remind him. For abandonment, if it affects the corrupt person, it is a form of da'wah (invitation to Islam) and reform.