Compensation in the joint

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Long story short, my friend has a car rental partnership with my brother.my friend lent two cars xenia on my brother,my friend got 5 million per month from borrowing these 2 cars. some time ago was reported missing one of these cars taken away tenants. my friends are confused about how to take the attitude, whether my friend should ask for compensation on my brother,or because he is his own brother then my friend does not need to ask for compensation.current search effort has been made,nd my friend still gets 5 million per month.then what if my friends will not want to work together again? please be input to this problem my friend. even better if the understanding of the law.