In the line "they found out that she had run away with a fellow," there is no specific figure of speech present. The statement is a literal description of someone discovering that a woman had eloped with another person.
A figure of speech involves the use of language in a non-literal way to create a particular effect or convey a deeper meaning. Examples of figures of speech include metaphors, similes, personification, hyperbole, and others.
However, the line you provided does not employ any such figurative language or comparison. It simply states a straightforward event or action.