To admit that their child has FAS is to admit to their inability to deny the addiction during pregnancy- would you honestly be open and chat with another parent about that?
I worked with a little boy at the very start of my career who very clearly has FAS. As part of our initial assessment, I asked the mother if she smoked, drank or did drugs during her pregnancy and she stated "no" and moved on. Clearly, she was lying. It took months of working with this child- testing, counseling, observing to build a relationship with the mother that would allow her to admit to the drinking, but it is so important to note here.. I HAD A REASON TO KNOW and she couldn't lie to me forever about it.
Bottom line- you don't have a reason to know the answer to that question. You are not family, you are not a clinician, you are a friend. Of course they are embarassed, ashamed, guilty- all of the above! Wouldn't you be?
Be tactful here and do nothing. If the child is struggling in school, the school will refer her for an evaluation and the truth will come out. You don't know for a fact that this child isn't being treated and frankly, it's not your business. How would you feel if the situation was reversed?