The greatest likelihood is that she has a hydrangea that is not fully hardy in her area or that it is being pruned at the wrong time. But it really is impossible to answer the question without knowing what kind of hydrangea she has.
I didn't know there was different kinds and she has never pruned it. We live in upstate NY.
There are many types. Unless she purchased it from a local nursery, I suspect that hardiness is the issue.
Your local extension office (found under government in your phone book) should be able to help you determine what type of hydrangea you have and suggest some solutions to your problem.