I suspect that LW’s husband has never adjusted to his dad’s current wife, considering she’s been around for a decade already and yet I’m feeling that “she’s not really part of the family, she’s just a new interloper” energy. It’s quite possible that the husband’s dad did one of those very common stunts where he remarried quickly because he didn’t like being alone, and LW’s husband is resenting her rather than his dad because that’s psychologically easier.

If that’s the case, yeah, I’m not surprised he doesn’t want her being given an honorific. And in that case he’s unlikely to be amenable to an alternative, because it isn’t about her having “Grandma” specifically, it’s about her having anything that denotes her as an honored member of the family who gets to have a special title.