Good idea. I would also encourage your son to keep a detailed diary of all interactions between her and also your experiences for any future court judgements.
Just an off topic mention- My mother was of similar personality as your DIL. When I was a toddler she caused the split up of my father to his 6 siblings. We didnt see most of them throughout my childhood. Then with the tool of Facebook about 60 years later a cousin from one of those aunties contacted me. Her mother, one that was estranged was 92yo with failing health. My sister and I made the trip to Newcastle to visit her and what a grand lady she was. The next year she passed away and I read out a poem about how "59 years was too long". The gap between her seeing me and then.
I have been estranged from mum for 9 years. She ruined my first wedding in 1985 then before marrying again in 2011 she threatened to ruin my wedding again. I even had to get a court order stopping her from attending the park where it would take place. She is 88yo now
These are the extents people have to go to in order to have a peaceful life- removing them from our lives if possible.
The mother of my children and I seperated and divorced when our girls were 7 and 4yo. That meant constant communication had to continue between us until the youngest was 18yo at which point I cut all contact with her. 14 years of mistreatment, nasty unwarranted friction. I was discouraged to go to parent and teacher nights (I went anyway), no slight changes to visitations (half an hour early not accepted) and so on just to get under my skin.
Anyway yes, your son is in for a long traumatic time but he can rise against the odds with support from you both and say to himself "I'm going to be the best part time dad my kids will ever have" which I said to myself in 1996 as I drove away from the family home. A week beforehand I'd tried taking my life. However she brainwashes her children against their father and you will come back as karma.
When my eldest reached 30yo just 2 years ago she got married. I wondered, had I taken my life who would have given her away? Then my daughter turned and said "thanks for being a great dad". My tears outnumbered everyone elses. I made it. Her biological mother wasnt there, my daughter had rejected her 12 years earlier for the same reasons. My wife is now "mum".
And you Feathers- you and your husband are great parents.
All the best