The full sentence would be something like: "You would have to ask her (if you want to know the answer)". But the second part can be omitted. "You would have to ask her" is more indirect and less forceful than "You have to ask her" or "You will have to ask her". They all have exactly the same meaning, though. In customer service for example, an employee might say to a customer, "I'm sorry, you would have to ask to my manager". This is a simply a more polite of saying "You have to ask my manager".

would have used to refer back to atime in the past from apoint of view in the future wouth used to refer to a situation that you can imagine happening

It is a conditional. The speaker could say "you have to ask her", but that is an imperative order to the listener, which may seem impolite. So instead, the speaker gives a choice: "you [if you want] have to ask her."

You are going to have to ask her or You will have to ask her. Would have - past tense. Will have - future tense. Let me know if you understand better!