。
She put a blanket over him.
Literal
She [topic-は] him [to-に] blanket [object-を] put-over-and-did-for-him.
毛布をかける ('put a blanket over') uses 掛ける ('hang, drape, apply') — one of the most polysemous verbs in Japanese, also showing up in アイロンをかける ('iron'), 電話をかける ('call'), 鍵をかける ('lock'). The benefactive ~てやる ('do for someone') here adds the caring nuance: she's not just putting a blanket on him, she's doing it for his comfort. やる (vs. あげる) suggests warm, casual familiarity — appropriate for a partner, child, or close friend.