You’re in the homeward stretch congratulations. Be patient and calm it might take some time and to-ing and fro-ing. Expect some fireworks - maybe a manipulative suicide threat from her ….. he needs to be prepared how to handle this - that he informs and adult / authorities ASAP - because if it’s a hoax she will be shamed - if it isn’t she will be helped.
Everything on your list means he is almost out the door - he knows everything cognitively - but this bit means there is still some emotional disconnection to complete:
“ DS is mad at her for accusing him. Like really mad.
He offered to speak to her friends to clarify but she threatened to dump him if he spoke to them.”
He now needs to learn to not be triggered by her to go running around after the dramas and histrionics she sets up - it’s now home for him to learn not to put his energy into this toxic system and to drop the rope.
I would be encouraging him to ignore / delete / block the flying monkey Dad - he is only self serving - he needs your son as a buffer because Miss Histrionic is coming back to him soon.
I would encourage him to give himself permission to turn of his phone to give her firm boundaries.
Don’t be too hard on him for not seeing it sooner, for being terrified within it, for being scared and unclear how to get out - not many people end up so entangled with such controlling and manipulative people - they are rare - he was unlucky to have got ensnared. But thanks to your calm and considered constant unconditional love he has been attracted back.
It’s not “if” it’s “when” - just think that it will all be resolved by NYE! Concede the birthday she will be all out to win that now - pick your battles and win the war.