The World Federation One Stop Fiqh

Ruling 2330

The guarantor and the creditor must be of the age of legal responsibility (bāligh), sane (ʿāqil), and no one must have compelled them [to enter into the suretyship agreement]. Furthermore, they must not be foolish with finances (safīh),2 and the creditor must not have been proclaimed bankrupt (mufallas). However, these conditions do not apply to a debtor; for example, if a person acts as guarantor for paying off the debt of a child, an insane person, or someone who is foolish with finances, it is valid.
2 Ruling 2091 provides further clarification of this term: it refers to someone who spends his wealth in futile tasks.