All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If you are obliged to pay the debt of the person you guaranteed, in case of his failure, according to the surety contract, and then he fails to pay it or dies, it would be invalid and insufficient for you to pay it from the Zakah of your property, since Zakah has its known recipients, no one else has a right in it, and it is unfit for the almsgiver to use it in mending his affairs or fulfilling his debts.
No analogy may be drawn between this question and the man who bore debt as mentioned in the famous narration of Qabeesah because that man bore a debt to serve a public benefit which returned upon the Muslims by saving their blood and making peace among them, and that is not the case in this question.
Allaah Knows best.