Having read @Justilou1's very wise words and reflected upon them overnight, I have to say that I agree wholeheartedly with what they have said. I thought that I could only give a perspective from the point of view of someone who parents a young woman with a personality disorder - but realised that I can probably also give another one.
I was a little older than your son, admittedly, being 18 at the time, but I got involved in a destructive, toxic relationship (with the NPD child's biological father, as it happens). He dripped poison into my ears in order to isolate me from my friends and family - and they all loathed him. But I thought I was in love and, consequently, would hear no wrong about him. My mother, in particular, was very out-spoken about the boy and her dislike of him. At that point, I was behaving like your son is, @workworkworkugh - spending every waking moment with him. I even dropped out of college for a while on his say-so. I ate with his family, went shopping with his mother and sister, helped his brother with his homework, had my own front door key. I went home, literally only to sleep.
Why? Because I knew my mother couldn't stand him, or any of his family (and she was right not to, by the way, but hindsight's a wonderful thing...). That was my main objective with the relationship from the very beginning of it - I knew my mother disliked him to a point where she'd suck air through her teeth if she so much as caught a whiff of his name on the breeze. He was my break-away relationship, my stab at independence. We've all had relationships like that, I think. Not all of them toxic, but still ones that define us as young adults in our own right, not just "Joe and Jane Bloggs' kid"...
I'm not saying that your son's doing what I did - but it could be that this is his way of starting to distance himself from the family a little. Break away enough to be recognised as his own person. The age is certainly about right - my own 16 year old son did similar last year with a girl who was wholly unsuitable for him (in my opinion). I bit my tongue and waited. If you do say something - like my mother did - then you do risk pushing him further into her willing embrace. I was fortunate in that I fell pregnant with my daughter, subconsciously realised it... and ran. But it took 14 months, and he'd cut me off from everyone who loved me. All because my mother kept telling me of her dislike for him and his family. I wanted to prove her wrong, I think. I genuinely thought he and I were going to be together forever - and that his insidious lies were the truth. I did see sense eventually, though, and I'm sure that your son will, too. It's simply that this girl isn't like "normal" bad partners - the death threat alone is evidence of that - and if she has a personality disorder, then chances are she'll be loathe to let your son out of her clutches.
The mother's emotive texts to your son, too, are more than a little creepy. It's almost as though she has a crush on your son and is encouraging his relationship with her daughter so that she can be vicarious about it. I could be wrong, but... a grown woman shouldn't be texting a teenage boy that way unless she gave birth to him! 