Of course, the suggestion that their child has "something wrong" is very unwelcome, every parent goes thru at least some amount of denial (well, its an only child so of course it takes longer etc).
While you are concerned and looking into the right direction for professional assistance, it will definitely be viewed harshly - only because of fears which they are going through.
I know 2 kids, brother and sister, the sister did not even say a word when she was 18 months... the only thing she said was mom. Her brother is now about 2 yrs old and he is delayed in his speech as well - he has very little vocabulary.
However, it's all God's gift - MA the girl is now 4 yrs old and is such a chatty cat!!
Her speech is MA very clear, she expresses herself very well and is basically smart!
I do not see her brother as often so I do not know his speech improvement.
So I think generally a doctor in USA really thoroughly examines and then recommends if needed. You can help by making sure they are going for there doctors visits regularly...