Is the husband's friend obligated to pay back the money he inadvertently lost belonging to the questioner's husband, and if the amount is substantial for him, is he considered among those in debt (gharimin) who are eligible for zakat funds, and is it permissible for the questioner to take from others' zakat to pay off the debt to her husband?

The husband's friend is considered trustworthy and is not liable for the money unless he transgresses or neglects its safekeeping, such as carelessly disposing of the money with the trash. In such a case, he is liable and must repay the money to its owner. If he is unable to repay, he is among those in debt (al-gharimin) to whom zakat (definition) may be given. It is not a condition for him to be penniless to receive from it. It is permissible to give him from zakat, but do not directly pay off his debt on his behalf without his permission.