。
She was glowing with love.
Literal
[topic-は] love [with-に] was-shining.
輝く ('to shine, glow') used metaphorically of a person describes a radiant, beaming state — the emotional source is marked with に, much as English uses 'with' in 'glow with happiness.' The に here is the cause/reason に, distinct from location or direction に. The ~ていた form gives the past progressive 'was glowing,' a sustained state rather than a single flash. The phrasing leans literary and would feel at home in a novel or romantic narrative.