One minute you won't name the rescue the next I'm supposed to have the details. 
I was asking YOU for the details as I thought they might possibly be pertinent. It seems I was right.
So your son went against RHU and rescue policy and agreement and advertised a dog he'd had in his care for a couple of days on Gumtree (which no decent rescue would do), free to a good home (ditto), without a homecheck (ditto) to a woman who lived in a block of flats? 
I said nothing about a bedsit
Some rescues will rehome to some flat-dwellers, on a case by case basis and after a homecheck. I would very, very much imagine that the rescue concerned acts similarly. That doesn't mean that they rehome to flat-dwellers as a matter of course so your comment is a bit misleading to those who aern't acquainted with rescue policy and practice.
Regardless, that does NOT provide any justification for your son to take it upon himself to place an unassessed dog belonging, at the end of the day, to a rescue, into an unchecked home of any sort.
Re your question - I would do whatever it takes to safeguard the dog. I told you that in (I think) my very first post. No kill, no excuses, no limits, no fear.
Would I have given the address of the person who had received the goods I'd stolen? I wouldn't have done as your son did in the first place and, as I have already said, neither would my own kids who are 14 and 15. But yes, had I had a complete personality change and become the type of utter prick who risked the owner and the dog's welfare I would have immediately given the rescue the address.
Your son is lucky that the rescue didn't go to the police.
Let's get this in context. The very vast majority of us in rescue are female. What was the sex of the person who allegedly thhreatened your 20 year old son?
And what did they actually say? Because that they will "castrate" someone doesn't seem to me to be the type of phrase that would generally roll off the tongue.
Finally, think about it... unchecked home. Unassessed dog. Female owner... no kids in her home? No visiting kids, no nieces, nephews, friends with kids, no chance of her getting pregnant, no-one at potential risk except her? You can promise me that can you?
And let's not forget the risk to the poor bloody dog of course. 