I'll echo normal person's good advice. It's hard because I'm sure you see your son's hurt, your feel your own, and you likely worry about your grandkids. Most of us here are living with strained, broken or dysfunctional relationships with people we love.
What has made you think she has BPD? Are both your son and DIL actively abusive, or is he passive?
Share more when you want to, and read other's posts here. This site has been a lifesaver for me.
