。
Her chest heaved.
Literal
She [topic-は] chest [object-を] caused-to-wave.
波打たせる is the causative form of 波打つ ('to undulate, ripple, beat like waves'), creating a transitive 'cause to undulate' from the intransitive 'undulate.' But the construction 自分の体の部位を~させる reads naturally not as 'forcing the chest to do X' but rather as the experiencer letting the body part express something — close to English 'her chest heaved' rather than 'she made her chest heave.' The chest heaving is a stock literary image for emotional turmoil — anger, panting after exertion, or held-back tears.