I haven't read the other pieces of advice yet, but if I was the friend (the one who is asking you), I would visit with the other mom and watch and observe a behavior that is particularly associated with autism and then I would say, "My friend, X (you), has a son that is autistic and he (does whatever). She gets therapy for him and found that it really helps if he is in a routine." (or whatever it is that helps). Then she could say, "If you are interested in talking to my friend about her experiences with her son, her name and number is...".
Then, leave it alone. If the mom doesn't jump on board and ask for help, let it go. The only reason I say this is, my brother dated a woman that had a 1 year old boy. I didn't know her that well (I lived out of state) and so I said something to my brother. Well, of course, he repeated to her and I am telling you, at the next family gathering, she chewed me out and up and down and said I didn't know a thing about her son. Of course, I didn't let it go. I sent my brother a list I found on the internet that said the top 10 signs of autism (this boy had 9 of them). By the time he was 3, I quit trying to "help" them because they weren't interested in listening to any advice. By the time he was four, he was diagnosed with OCD. Finally at 4.5 he was diagnosed with autism. It is sad and disheartening to think about how much help he could have gotten from age 1 to 4.5 if they had been willing to have an open mind, but they weren't. She and my brother broke up around this time, so I have no idea how he is doing now, but I imagine not well since she was not interested in getting him any help.
Good luck!
L.