Your husband is thinking of his mom, and of his OWN feelings in this. You know what he's not thinking about? Your family's safety and well-being and stability. He needs to quit being delusional and selfish and cut that woman out of your lives until she goes through a rehab program and makes drastic changes in her life. She is not safe to have around your children or house. She will damage your children.
My MIL is currently a prescription drug abuser, but when my husband was a kid to when he was 16 she did meth. We have given her many, many chances. At one point sshe stayed sober for 8 years and during that time we had reconciled and allowed her to be a grandma. Then she started abusing her prescription painkillers for her back problem (while she lived with us!), which I still think was mostly faked in order to get the painkillers. Drug addict behavior is not good for kids, we had so many problems with her before we realized she was abusing her meds. We cut her out for the last time (and got a restraining order when she wouldn't quit harassing us after we moved) after she attacked me in front of my son and called the police on me in my own house, stating that I was an intruder and was armed. My son busted out of the side door while the cops were interviewing me, and startled them causing 2 of the 4 there to draw weapons. That bitch could have gotten my son shot.
You know who was hurt the most by grandma being a druggie piece of shit? My son. His heart was absolutely broken by that shitbag, and he does not even want to be reminded of her existence anymore. They used to be best buds.
You need to tell your husband to protect his family, which includes you and the kids. Not her, anymore. He is being selfish by trying to make this happen, and it's going to damage your kids and expose them to things they should never be exposed to, which has probably happened already to some extent.