J.S.
There's a saying that goes like this: "When you hear hoofbeats behind you, don't expect to see a zebra." This relates medically. It means that someone has received a surprising diagnosis.
Your friend's son has Autism, and it was likely initially a surprising diagnosis.
But let's take this a little further. Since her son has Autism, it consumes much of her life. Autism doesn't turn off at 5:00. It doesn't turn off at bed time. There's no off switch at all. ASD becomes a way of life. And yes, there's another saying that when you know one person with ASD, all that means is that you know a person with ASD and that you don't know all about ASD.
She could be in a room full of horses and all she sees is zebras. I know what that's like. My middle daughter has Autism.
However, let me tell you that looking back in hindsight with what I know about ASD and looking back at the warning signs, it was very obvious that she had ASD when she was 18 months old. It was even more obvious when she was 24 months old. She had global developmental delays which drove the social delays and eccentricities home... and really, that's the key. There's more to look at than social cues being missed and not wanting to play with others and enjoying organizing things. There are global developmental concerns. There may be regressions in some areas. There would be significant delays consistently in several areas, not necessarily with speech. There could be sensory processing issues.
So while your friend could be seeing nothing but zebras in a room full of horses, she could also be seeing some familiar signs. I wish to God that someone had mentioned their concerns to me. There WERE people who had concerns at 18 months old and never said anything to me until after I decided to take her myself at 4 1/2 years old. "Oh, yeah, I heavily suspected Autism but I was afraid of how you would react."
It's better that she said something. Take your son to Birth To Three or whatever Early Intervention is called at the school system in your town and you can have him evaluated for free. It can't hurt. He is NOT too young.