Here's what I got from renowned speech therapist Dr. Sally Ward in BabyTalk, is a book named after her program, which helps babies and young children who are behind (but is also extremely good for all little ones in helping them to develop fantastic speech skills, which of course is crucial to learning, socialization, and other good things. The slow-developing children in her programs generally ended up with high IQs and even a substantial number of them gifted! The control group of late talkers did not; many ended up in the below-average range.)
(HAVE YOUR FRIEND BUY BABYTALK. IT WILL MAKE ALL THE DIFFERENCE FOR HER SON.)
Tell your friend that t.v. is a major culprit in late talking/speech problems. Tell her that she and her husband need to make a commitment to turning it off and leaving it off while he is awake (and making sure it is quiet enough that he doesn't hear it in his sleep).
(You'd think that exposure to all of the talking on t.v. would be good for them, but it is a passive experience, so the connections aren't really firing in their brains while they're watching. This is a key point. They need to be hearing language all around them, but from people who matter, and they must have the interaction.)
Tell her that they need to NOT do the very common "can you say [word]? Say [word]! Say [woooooord]! [Word]!" This actually confuses babies and makes them shut down - I think due to feeling pressured. They probably don't understand the dynamic - they don't observe adults talking to one another like that, after all.
This might sound entirely off-topic but, most importantly, she or her husband need to spend at least half an hour with him every day (they can take turns but it needs to be only one parent at a time so the child can focus), in which they play together in very easy but specific ways. I don't know what the program is for babies of his age; I've only looked a few months ahead for my 7-month old. I know that allowing babies and small children to focus on whatever they want to focus on is really important (this is hugely important for minimizing or preventing add/adhd, too, by the way; their attention span is going to be short in early childhood, but every time we're like, "ok, let's focus, let's turn the page," (for example) that fragments their attention even further, and makes learning more difficult. Let them "be distracted" and bang the book on the bed or whatever. This is probably true for 16-month olds, as well.
Tell your friend to take heart. Apparently, you start to see results with this program quite quickly, and he'll be caught up to at least the average development for his age (probably will exceed it) within a couple of months.
L.