First off death in a dream rarely means death unless you have premonition abilities.
Usually death means a closure of some sort , like a an old chapter of life ends another opens.
The ferret overboard dream I would say is ... something what is most important to her in life not necessarily the ferret but something is just out of her reach , like the ferret she can see it but just cant reach or attain it. OR maybe there is something she needs to let go of but doesnt want to.
Now what was the water like if it was rough ride or smooth sailing also applies to her life.
Dying of cancer , well again I would say its highly unlikely but if she has any concerns or notices anything odd see a doctor. ( covering my arse there lol) but this brings me back to the ferret overboard where I said is there something she needs to let go of , something may be eating away at her ( symbolism , like a cancer)
Hope this helps Vamp , let me know how I went.