To start, don't panic, she could simply miss them. People dream about dead loved ones after a passing, or at a time of celebration [the holidays, a birthday, wedding] because they miss seeing them at these events.
Dreams can be funny. I usually dream about a pet or loved ones death a week prior, but when I was in my teens there was about a year where I dreamt my grandfather would die and luckily he never did. Still alive and well in fact, so don't believe your dreams 100% of the time.
To be safe, she could go to her doctor for a full physical and some tests run to make sure she's fine. It could be a self-fulfilling prophecy if she stresses herself over dying she ends up dying. Best to relax, speak with a doctor, and tell her it was just a dream.