This simple guide to normal early language development:
13 months of age: first words emerge (e.g. "here", "mama", "bye bye", "kitty")
17 months of age: fifty word vocabulary
18 months of age: First two-word combinations (e.g. "more juice", "here kitty", "cup floor")
24 months (2 years) of age: Average length of sentence is now two words with "-ing" emerging (e.g. "playing", "hiding", "running")
30 months of age: Average length of sentence is now 3.1 words with "is" emerging (e.g. "My car's gone!")
36 months (3 years) of age: Average length of sentence is 4.1 words with indirect requests emerging (e.g. "Can I have some cookies?")
Should they be worried? We wouldn't expect everyone to understand a 2 year old so he might be saying words which are just unclear. We'd want to look at his interaction skills: does he have shared attention, make eye contact? We'd also want to know about his understanding: does he follow simple instructions or rather does he follow visual clues? There's a difference to understanding words or understanding situations. Think about if you were in the middle of Russia, you'd cope by following non-verbal cues and looking for any clues to what was meant. If you say, lets go in the car and they go to the door, if you have your car keys in your hand and your coat on, they may NOT be listening to the words.
Talking to his mother, I'm not too worried and I've given her some suggestions to try. He was premature and has been a little late with all his milestones. He's also a very passive child. We'll have a look at the suggestions next time for developing reasons, means and opportunities to communicate.