Does the car driver and the friend of the passenger who was riding the motorcycle in the opposite direction bear sin, burden, or blood money due to the amputation of the passenger's leg as a result of the accident, and how can they absolve themselves of that?

If the cause of the accident was driving on the wrong side of the road, then the motorcyclist is at fault and bears the consequences of the accident. The competent authorities are responsible for identifying the wrongdoer and determining the percentage of their fault.

An offense committed by mistake is not inherently sinful, but a person may be sinful if the cause of the offense arose from negligence or carelessness with a high probability of harm.

Blood money (Diyyah) is obligatory in cases of error. For a single male, half of the blood money is borne by the offender's 'Aqilah (male relatives responsible for paying blood money). According to the Hanafis and Malikis, the offender shares the responsibility of bearing the blood money for an error with the 'Aqilah. This differs from the Shafi'is and those who agree with them, who believe that there is no liability for blood money on the part of the mistaken offender.