。
She made him happy.
Literal
She [topic-は] him [object-を] happy [into-に] did.
The Xを Yにする pattern means 'to make X (into) Y' — agentive change. With na-adjectives or nouns, the resulting state Y attaches via に. Compare with the intransitive partner Xが Yになる ('X becomes Y'). The polite ました ending suggests this is being told as a small narrative or observation. A short, simple sentence that demonstrates a cornerstone construction for talking about causing change.