You asked about others here who have had similar experiences. I just had my son, who was an ideal kid, go estranged due to friction between me and my husband and him and his new wife. It wasn’t about drugs or cheating. It was about individuation ‘on steroids’. That’s the only sense I can make of it.

Since you love your son and grandchildren, my advice to you is to tread lightly and be careful. The most important thing is that you have influence on those kids by getting to spend time with them. Be there for support. I’m sure your son knows how you feel about his toxic relationship. I’m sure he feels the same. It’ll play out however it will.