N.D.
It seems everyone agrees that you should not tell the mom that you noticed something AFTER the evaluation. But I think you are asking how you might mention something to another mother if you notice something. You might comment something like, "Boy is so cute, but he is very quiet. Is he shy?" At another time you might say, "Boy is so serious, he doesnt smile very much does he?" Just make of hand comments that might alert the mother without coming flat out and suggesting anything is wrong. But most moms compare their children to others and suspect there might be a problem, but are avoiding getting a diagnosis.
Personally I feel there is an alarming increase in DIAGNOSING children with the "Autism SPECTRUM". Im 68 years old and years ago a child had to be completely withdrawn to be called autistic. Now it seems any child that is the slightest bit shy or speech delayed is plopped on the "spectrum." I imagine Bill Gates would have been labelled very quickly. Also any kid with boundless energy is ADHD and put on meds. Imagine where Shaun White would be today on Ritalin.
But thats my personal soap box, so Im jumping down now and wishing you blessings for being such a caring friend.