1. It was sad to see her the victim of continual pain.
The underlined part here is the object complement of "her" (i.e. it presents an attribute of "her").
This isn't a very frequent structure; other examples are:
2. We appointed him chairman.
3. She made me a very happy man.
Cf. verbs that take a subject complement, which presents an attribute of the subject:
4. She looks an interesting person.
5. It tastes rather strange.
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