Do the bonds of trust signed by the friend on behalf of his brother constitute sufficient proof of debt, and is the plaintiff required to swear an oath to prove that the amount recorded therein is a debt owed by the friend and not by his brother?

As long as the guarantor has pledged to repay the money if the guaranteed defaults, it is permissible to demand the debt from him. If the guaranteed is unable to pay, the guarantor is obliged to perform, and then he can reclaim the debt from the guaranteed. If the guaranteed goes bankrupt, the debt remains his responsibility until he is able to settle it. There is no harm in lying if it is the only means to attain justice and prevent its loss, but it is preferable to resort to dissimulation (Tawriya) to avoid lying. If the one taking an oath is compelled to swear before a judge, he may resort to dissimulation or intend a specific meaning for his oath, especially if he is oppressed.