Is Zakat obligatory on money that my brother lent to his friends, and he does not know when it will be repaid?

The zakat on loaned money does not fall away from its owner. Zakat is obligatory upon him as long as his ownership has not ceased due to the loan. If the loan is to a wealthy person, acknowledged by him, and he is capable of repayment, then your brother must pay zakat on it every year, because it is a debt expected to be repaid, and it is considered like money in his possession according to the Shafi'is. As for the Hanbalis, the debt is zakatable for all past years once it is received, and he is not required to pay zakat on it before receiving it.