Look, the best thing your husband can do is realize that bridge burned the minute he put a boundary, which was that whatever she had to tell him she could do in front of you( a very reasonable boundary), and she chose to go NC because of that.

His father is fucked up but his mother isn't far away from being like that either because she exploited over her son giving his wife a place. Going NC because of that is next level.

I don't think your husband misses his mother. I think he'll always grieve the fact that he never got a normal set of parents like many of his coworkers, friends, cousins... have. That will always hurt him, BUT right now he's at such low point that he's "self-harming" by trying to keep in contact with his mother just so that she can hurt him with her indifference. He's punishing himself for whatever reason by reaching out to his mother knowing that he's gonna get nothing but hurt as to how little she makes him feel.

Also, I think it's unhealthy and fucked up that he's got in contact with a rapist just because he wants one parent at least. You cannot seek for some sort of connection with your mother and then turn around and try it with the man that raped her... I know he did it because he's in a dark place and isn't thinking straight but please..