Parents of hearing impaired children, should they learn signing?

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I have a 7 year old grand daughter and she is hearing and speech impaired and also Autistic. She goes to a special school that is for these issues and she is learning to use sign language. I get her every summer since she was 2 1/2 years old. Her mother does have health issues and dad works and is a vol. fire fighter for two companies. She has all kinds of electronics to entertain her at home, DS system, DVD and TV, Leapster, ect. She is an only child and they live 5 1/2 hours from me, so I enjoy my time with her. I have taught her to walk at 2 1/2 years and also potty trained her at 4 1/2 years old. I was excited when I picked her up this year and she taught ME how to sign colors. We have been working on signing, I got her involved with a summer deaf camp in the area here also. So, with all the background information, here is my question: Do you think parents that have a hearing impaired child should learn signing? Her parents won't and her mom's reason was, "She never asks me for anything." My grand daughter, step grand daughter actually, enjoys that I am learning how to sign and that we started doing it together. She is able to start to ask me for things that she wants or needs. When I potty trained her the first sign I taught her was "potty" so that she would be able to tell people when she has to go. My thing is, how will they deal with things like, if someone would hurt her, or if she would start her period early in life? How would they make her understand. Then there is the issue of her being Autistic and not understanding somethings anyways, like she didn't know what a kitchen counter was, she would put everything in the sink. Do, what would you do or say to the parents? I have brought this up to them and even bought a book before that was never even used and they probably have lost it by now, that was 2 or 3 years ago. Thanks for your time.