。
She helped the old man cross the road.
Literal
She [topic-は] old-person [target-に] road [object-を] made-cross gave-as-favor.
A layered construction: 渡らせて (causative of 渡る) + あげた (benefactive). The combination ~させてあげる means 'to let/help someone do something as a favor.' The elderly person is both the causee (marked with に) and the beneficiary. This stacking of causative + benefactive is a key intermediate grammar pattern.