to my mind, not in the way a physical illness has symptoms, if you have a cold, flu, you are likely to get a runny nose, tickly cough, but with this it can come out of the blue. If someone you care about dies, its shock, despair, anger and so forth, but calling it a nervous breakdown is a catch all by the medical profession, there are so many types of mental illness, i have witnessed it first hand, and in my own regard, that also includes a number of people i have met with serious mental health problems along the way.
A relative went undiagnosed for years with the mental illness he has. Suggest that she see someone sooner than later, a good listening doctor, who can put her onto to a counsellor, and take it from there. If you are not in UK then thats a tough one, as here at least you will see someone without having to pay, unless someone here goes private. She should see someone though, before the problem escalates.