Because it is clearly stipulated in the joint guarantee agreement you signed, the three of you shall bear joint liability. If Zhang doesn't pay back, both you and Li have to bear Zhang's repayment obligation, which has nothing to do with the loan officer. In fact, this agreement is really unfair to people who repay normally, so when forming a joint guarantee team, we must choose honesty. What you can do is to persuade Zhang to repay normally as a friend and be reasonable.