I think there are several different issues at play here.
First, is the R word offensive? It has definitely fallen out of favor in recent years, but for decades it was the clinical term to describe someone with an intellectual disability. It replaced "idiot," "imbecile" and "moron," which were the clinical terms for different degrees of intellectual disability a century ago. Now those words are used exclusively as insults and never applied to actual developmentally disabled people.
It lives on in the names of some agencies (around here services for this population are provided by the local ARC -- Association for Retarded Citizens, although the C word used to stand for Children) and in casual conversation.
OP has stated that she dislikes the word, so if she used it in conversation with her friend, she was probably parroting/quoting her daughter, who was worried about having a "retarded" child. And I can see a worried mom saying, "Oh, no, what if he's retarded?" not the less offensive "Oh, no, what if he's developmentally disabled?" And OP's friend might have thought it kinder to use the word OP used, rather than try to "correct" her by rephrasing it with a more PC term ("Well, if he turns out to be developmentally delayed ...") that might have come across as getting judgmental on OP about her word usage.
As for OP's friend's comment ... how many threads have we had here about people attempting to comfort others and sticking their feet firmly in their mouths? She was probably thinking something like "Well, it will be a hard road, but your DD is a good mom, loves her son and it will be OK" ... it just didn't come out that way.
BTW, this may all be moot. I don't have children, but I've been around enough of them over the years to know that nonverbal at 18 months is no cause for panic or fearing the worst. As several PP's who do have children have pointed out, some kids just take their time with certain things.