La vs lui

Question

In the following example, I am struggling to understand why we must use la and not lui? To me, it sounds like the sentence requires an indirect object pronoun, because the question "What" is not answered in response to the "must", which is the verb in this sentence. I use the "what" test to determine if there is a direct object in the sentence. With this sentence, should I consider "what must they warn" as my question, or "what must they do". Apologies if my line of thought is completely skewed but it seems to work in most cases.

Does Julie know? We must warn her ,

- Julie est au courant ? Il faut lui prévenir,- Julie est au courant ? Il faut la prévenir,

Accepted Answer

The ‘what’ or ‘who’, is not the question that decides between using a direct object ‘la’, or an indirect object ‘lui’.  You have already identified that an object is required - it is determining which verb and whether a direct object or indirect object is needed that you are stuck on. 
In this example, is it ‘who’ or ‘to whom’ that you need to follow the verb with ?
For that you need to identify the correct verb, and know the nature of the verb. The verb taking the object pronoun is ‘prévenir (qqn)’, no preposition after the verb before a noun, a direct object pronoun is required - ‘la’ for ‘Julie’. In this example it is the same as in English, but that is not always the case !
 The lesson below should help further with the principles, which are also covered in the lesson you linked. Online dictionaries are helpful for the verb expressions. Verb lists help for reference, but I think it is better long term to learn verbs in context.
https://progress.lawlessfrench.com/revision/grammar/when-to-use-lui-and-leur-to-replace-specific-people-with-him-her-and-them-indirect-object-pronouns 
https://www.wordreference.com/fren/prévenir 
https://www.lawlessfrench.com/grammar/verbs-with-a/