Honestly, only you can judge what sort of relationship you have with your friend and whether it would be appropriate to say anything. I can think of 2 friends/relatives with whom I think I have a close and open enough relationship to bring up something that sensitive.
Does your friend take her LO to all their well-baby appointments? If so, then I think I would not say anything. How exactly do you envision the conversation going? "Excuse me, friend, but I think there's something wrong with your LO." I really just can't imagine it going well, and I don't think it's necessarily your place to tell her you think there's something wrong with her child. Maybe the things she says he can do are things he does at home where he feels comfortable, but not around strangers.
If you really feel you have to say something, maybe you could do it in a casual conversation if something comes up that's not normal, like "Oh, have you mentioned to your pedi that he's not speaking yet? I know lots of kids start speaking later, but mine said that if LO hadn't started speaking by 18 months that we would run a few tests to rule out hearing problems or other issues." Then you're not being confrontational about it. I don't know, though. I honestly think it would be best to just let it be.