父が友人の保証人となった。

。

My father became a guarantor for his friend.

Literal

Father [subject-が] friend's guarantor [as-と] became.

保証人 (ほしょうにん) = 'guarantor' — a person who guarantees another's debt or obligation. ~となった means 'became (as)' — the と marks the resulting role. Being a guarantor carries significant financial risk in Japan.