Your son, as the father, can do something, but this is not your battle to fight.
First of all, there is NO test at birth for autism, and no single "test" is going to provide that diagnois, not at birth, and not at anytime after birth. So I don't know what she had done, or what your understanding of what this child has is, but that is just a total misunderstanding of autism on every level. Autism is diagnosed based on a very lenghty multifactored evaluation by a Developmental Pediatrican, who calls in ENT's Speech Therapists, Occupational Therapists, Geneticists, Physical Therapists, Neurolgists, Phychologists, Optiomologists, and Neuropsychologists (and any other profession they need) to make a comprehensive diagnosis and a full treatment plan. One thing your son can do is to take his children to see a Developmental Pediatrician for full evaluations himself, such that they are, or are not, diagnosed properly with any developmental issue. If that has not been done, neither child is properly diagnosed, nor have they had any thing ruled out.
I don't mean to offend in anyway, but as a grandma, MIL, or what ever your relationship is with the Mom, you are already on that automatic shakey ground of assumed "this girl is not good enough for my son" syndrome, so she may tune you out, no matter how right you may or may not be. You should really be leaving this to your son, this is his fight.
Some people do have mental illnesses that cause them to see illness in their children, but this is rare. If your son beleives this to be the case, and he is married to this woman, then it is his responsiblity as her husband to help her get the proper treatment for her illness and to get well. If he is not this woman's husband, then he has a responisbility to use the legal system to get custody of his children and insist that his children's mother seek treatment for her illness so that his children are protected.
Finally, as a mother with children who have autism, you may be way off base. Find out about autistic spectrum disorders. ASD ranges from mild to severe, and from experience, the vast majority of the population has no idea what they do not know about autism and what it really looks like. My children walk and talk and look like most other children, but they have autsim. I can tell you this, some children do get the proper diagnosis, and some parents do work very hard with their kids so that they don't look obviously disabled, and our reward for all that hard work making our kids seem indistinguishable from typical children at first glance, is that when they have a melt down or an out burst, the assumption of most underinformed people is that we have never bothered to dicipline our typical kid. That is not the case at all, when the exact opposite is happening, and what you are seeing is a child whose parent has done tons and tons of work with them, and the child just had a slip into autistic, and expected, behaviors for them. There is nothing any one of us can really do about it, but it happens over and over, and you may be seeing that very thing, and really owe your grandchildren's mother a world of thanks for making your grand child appear "normal" most of the time, instead of scorn that she sees what is not typical in an attempt to get them help.
I don't know which of these scenerios applies to you, but I have seen them all, and as a woman who does not spend much time with my own family because they don't get it, I can say that it happens more often than you think.
I hope that this is helpful to you and you can look at all the sides so that your son does the best thing possible for your grandchildren, what ever that is.
M.