Astudent says that if the hypothesis is false, an argument cannot be valid. how do you respond ?

If a student tells me that an argument that has a false hypothesis cannot be valid, I would reply that we need to look carefully at the meaning of validity in the context of logic.  In everyday speech, we tend to use “valid” to mean the same thing as “true” or “accurate.”  In logic, this is not the way the term is used.