。
She became a guarantor for his loan.
Literal
She [topic-は] his [genitive-の] debt [genitive-の] guarantor [into-に] became.
保証人 ('guarantor, cosigner') is a critical role in Japanese society — required for many contracts (apartment rentals, large loans, sometimes school enrollment). Becoming someone's 保証人 is a significant act of trust and responsibility, since the guarantor bears legal responsibility if the principal defaults. The XになるY structure ('become Y') here describes the role she takes on. 借金 ('debt, loan') usually implies money owed; for formal loans the term ローン (loanword) is also common.