Speech develops for different children at different ages. It has been reported that Einstein was turned four years old before he actually spoke.
My own twins, with three not much older siblings - did not talk at all when they were two years and three months old. Then over a few weeks went directly from talking into full sentence constructed speech.
The nursery will indicate if they consider a child needs to be referred. One my g.sons had what was eventually diagnosed as something called 'backspeech'. Nobody could understand anything he said (this was at 4 and 5 yrs old). He would get very angry and frustrated at this lack on other people, even the other children at nursery and school could not comprehend him. Wonderful Speech and Language therapists worked with him, and within less than a year he was talking normally. At 10years old he speaks beautifully (is usually used as the Narrator for school plays). Still can only work out how a new word is pronounced if he sees it written down.
Does sound as if the parents have their own ideas of good parenting - in the long run this will sort itself out. Are they considering a further child? that might help. But do not interfere- just let the parents know that if they ever do feel they wish to talk about anything related to their child, you are ready and willing to listen.
Baby Reindeer - anyone watched it?