"Ella lo vio la semana pasada."
Translation:She saw him last week.
Has to do with Direct vs. Indirect Pronouns. Ask the question "What did [Subject] [Verb]?" In this case, "What did she see?" She saw him, so it's a direct Pronoun and you use Lo or La.
Indirect pronoun has to do with who recieved the direct object. She gave him a book. What did she give? Direct object=a book Who recieved the book? Indirect object=him