This is a very sticky situation. It's hard to ignore the problem, because if your observations lead to the child getting help, that's great. The key is to broach the subject in a caring, non-judgmental way and never use the word "autism." Instead, share your observations with your friend, such as "I notice when we play he doesn't really engage with me. Is that typical for him to not respond to other people?"
This acknowledges your friend as the expert on her child, while still mentioning your concerns. It may be that the child just takes longer to warm up to people other than Mommy and Daddy, which is totally normal. If your friend responds defensively, or by saying that she is not worried about it, and that he's fine with her and her husband, then there's not a lot you can do at this point. Most parents with a child who shows the kind of behaviors you describe know something is not right. They may just not be ready to publicly address it. But your observation may be just the validation she needs to get her child evaluated by an expert (although she may not tell you that). Either way, you've done all you can do as a friend by making your concerns known.
You should also consider the kind of relationship you have with your friend before talking to her. Would she want you to bring your concerns to her attention or will she consider it intrusive? Hopefully she'll react well and recognize that you care for her and her son. However, you might also want to be prepared for the possibility that she will be upset by your comments and react negatively, affecting your relationship.