Have the grandparents looked for a support group for families of drug users? That might ease some of the stress in the marriage and they probably can direct her to other support groups too. Couples counseling may also help. I think they need to reach some sort of agreement about how they intend to deal with their son and stick to it. She might also want to look for support groups for families of children with disabilities because that presents a whole new set of challenges that other support groups may not know how to address.
I have a friend who’s going through this with her brother. Their mother recently got custody of the grandchildren. She said that they were advised to quit bailing him out of problems – don’t give him money or pay his bills, etc. – because the only way for a user to quit is because they want to, and the reason they want to is generally because it’s negatively impacting their life in a way they don’t like. Basically, they have to hit rock bottom and they can’t when they have a safety net. When OP and her husband are gone, the only safety net their son will have will be their grandson. Better to let him fail now, when they’re around to protect their grandson and model coping strategies for him.