I may have put it differently, but this was my thought, also.
OP, you say that DS is now behaving completely out of character, so perhaps the GF is the source of the change. Her refusal to visit over Christmas sounds like an attempt to coerce.
She is five years older, so around 30, yet not contributing anything towards the deposit for their house? Why? DH is older than me; during our engagement and while we were working apart in the first year of our marriage I lived very thriftily so I could contribute almost equally to our first deposit. It was a point of pride.
Could DS be in thrall to this woman? Could she be looking at the sports car, hearing about the private education, seeing the good job and falling in love with his money as much as anything? Not that there is much parents can do about it, sadly.
My only real difference of opinion with @Mrswhatsit40 is that I think you and DS’ father are the wrong people to try opening his eyes to this, if it is indeed correct. Does your DD know the GF and if so, what does she think of her? DS might listen more readily to his sister.