Wow, you've gotten so many answers I wasn't even considering posting an answer here. After reading most of the other posts, I am now compelled to offer my thoughts.
First, I AM the mother of a special needs child. My child does not have the same issues as your friends child, but parents of children with special needs are fiercely protective of our children, so I have some idea what she is going through and how she feels.
I will not bore you with the long medical history of my son, that doesn't matter here. But I will share what happens when well meaning "friends" put their noses where they don't belong.
Many posters have suggested you back away from the friendship if you can't be supportive without being jugemental. I will counter that advice with this: when you back away, you will hurt her as much or more than you would hurt her with your opinions of her parenting. She is a smart woman, she will know you don't approve of the way she parents her child and you will do more damage by walking away.
Some have posted that you should confront her, in a nice way, and offer advice on better methods of parenting. Do not share your opinions with her, it serves no purpose. She is navigating a new and scary world. Ask her how her son is doing. Ask her to explain her son's diagnosis. She needs to talk about it with someone who will listen. She will mourn the loss of a "normal" child and she will need a friend who loves her.
Some mentioned the possibility that the childs issues were somehow her fault. DO NOT UNDER ANY CIRCUMSTANCES BLAME HER FOR HER SONS ISSUES. Not to her face, or behind her back. She already has guilt from not being able to get pregnant, then losing a baby, and now having a son with issues. She will blame herself many more times than others will blame her. The very last thing she needs is for others to suggest it's her fault. Ask her how she is doing, but please do it honestly. See if you can set aside your feelings and opinions long enough to be a support to her. It is hard, I too have friends who make me a little crazy with their constant rant about one thing or another, so belive me, I can see it from both sides.
Let her know you love her and that she can trust you, (if in fact that is the way you feel). If you offer to watch her child to give her a break, make sure you ask her if there are any special instructions for his care. The biggest mistake you can make with a special needs child is to put your own spin on the child's care. Please do things the same way his mother does them. I can tell you from personal experience that when someone else watches my son and tries to "teach him to behave", the fall out from that short period of time makes me want to scream at the person who has been watching him.
Special needs children can be the loves of your life. Raising them can be taxing, emotionally draining, and just plain hard. Those parents who raise special needs children need the love and support of true friends. If you can be that person, bless you. If you can't, know that when you back away from this relationship, you will hurt her, and the friendship will be damaged, never to be the same again. It is your choice, obviously. Whatever you choose to do, do it 100%. Either be there for her, and really be there, or walk away. If you choose to walk away, then walk, and be gone.
Best of luck to you.