If you must give advice, do as Monica suggested regarding your ex, and tell your son to grow up and realise that being a husband demands some loyalty to his wife.
Irrespective of what he privately thinks or has said to her about the occurence, he should tell his father that he is not prepared to listen to criticism of his wife from anyone. If his father is so discontent with his DIL then he takes the matter up with HER again, and only with her.
Having said all this you add to both your ex and your son, that you do not wish to be involved in this quarrel, nor to hear any more about it. And stick to this.
To think that London, or anywhere else for that matter, does not belong to any one demographic