。
She is proud of her son.
Literal
She [topic-は] her own son [object-を] pride [as-に] is-thinking.
自慢に思う uses 思う ('to think, regard') with 自慢 ('source of pride') marked by に ('to think of X as a pride') — slightly more inward and reflective than 自慢にする ('treat as a pride'). The two patterns hover close in meaning; に思う leans toward subjective regard and personal feeling, while にする leans toward outward attitude and behavior.