。
She became a true friend to me.
Literal
She [topic-は] my sincere friend [into-に] became.
誠実な ('sincere, honest, faithful') is a な-adjective; the な before 友 (here in the literary one-character form) is the standard linker. The frame Xになる ('become X') is one of Japanese's most basic change-of-state expressions — the noun being transformed-into takes に. 友 by itself is more literary/elevated than the everyday 友達 or 友人 — fitting the register of the formal な-adjective 誠実な.