Nothing in particular. Losing a parent is a common fear for most of us, and since dreams are made of memories and bits from our subconscious mind that "recycle" while we sleep and are often oddly mixed up, that idea will come to the surface now and then. If she dreams about it repeatedly, it may be on her mind more than usual and you may want to talk to her about it, in case she is worried about something. But if only once, now and then, it's just the same-old same-old, cycling through. We don't always dream of things we WANT, you of course know.