Chances are, she knows her toddler doesn't do all the things that other kids his age do, and the last thing she needs is for you to compare her child to yours -- especially if you're not a doctor or certified child-development expert. If you really feel there's a problem, say something like "You know, I've been reading up on what to expect when my son is your child's age, and I'm wondering if your son does any of them," then mention a few. If she hasn't already noticed something may be wrong, this convo may get her to talk to her pediatrician. But it's probably best to bring this up only once. After that, it's up to her to decide what to do with her child -- even if you don't agree with it, don't think it's adequate, or would do something totally different if you were in her shoes.
Q. I think something is wrong with my friend's 2 1/2-year-old son; he won't play with kids his age, and my 18-month-old speaks better than he does. Should I talk to her?