。
She didn't agree to his coming.
Literal
She [topic-は] he [subject-が] come [nominalizer-こと] [to-に] did-not-agree.
賛成する ('agree to, approve of') takes に for what is being agreed to. The clause 彼が来る ('he comes') is nominalized with こと, and the whole nominalized event is what she didn't agree to. Note が — not は — marks the embedded subject; embedded subjects almost always take が.