Many old people are simply grumpy... so perhaps that's just how things are and you must live with it.
In alternative, have you ever discussed this with your mother? Perhaps an honest talk, avoiding accusations and blaming, about the situation could do some good. I doubt it solve things for good, but it may help your mother. She seems just to be adding suffering to pre existing suffering, meaning, she solves nothing and only makes things worse than they are. I'm sure she doesn't want to make it worse, but perhaps that's the end result.
A hint would be avoiding religion in this conversation. She has no certainty about God's existence, let alone heaven. Keep it human, here and now, action and consequence and show her how her behavior only makes things worse and solves nothing. Nevertheless, if she keeps acting like that, perhaps you could consider taking your father to live with you for some time. Call it a prolonged holidays and keep him with you for a couple of months. She will probably start missing him and stop acting like that. If she doesn't, there's a saying that goes more or less like "better alone than in nasty company" here in my country. If she treats you father badly, why does he put up with it and doesn't get a divorce? People are free to live their lives with happiness, even when having 80. Anyway, seems to me your mother needs a reality check to understand what she's been doing. Spending some time managing loneliness on her own may be the best remedy.