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".