Having myself a severely autistic little brother (well, little is a relative term: he was born when I was 4, now he is a good 6cm taller than me), I can indeed tell that even a single autistic child can be more than a handful.
I didn't see it then, being a child myself, but our parents really struggled. Not so much in the monetary sense, but in the parenthood sense. They've never been ashamed of him, neither have I or my big brother. But taking care of his needs have caused us severe headaches in the past.
Trying to imagine four more of him... I just can't do it. I can't see how I could deal with all of them equally, take care of them equally and find solutions to ensure their happiness. Not to the level they deserve, not to have the best possible chance at happiness in life.
The mother... it isn't my place to say if what she is doing is moral or not. To be honest, I truly don't know. Both positions could be argued for and against.
All I can say while remaining true to my personal view on life, but simultaneously thinking of my little brother and the stuff we've gone trough to include him in our daily life as a brother should and now that he is an adult to give him the best chance at happy life...
is that the mother, should she wish for more children, perhaps think things trough. To at least have some foresight, some plan of future, should she decide to have more children and in the case they too have severe autism.
It isn't my moral right to critize her potential decisions regarding having (or not having) more children. But what I feel I can do, is question her preparedness for the possible case that her potential future children might have severe autism as well.