Hi Patallen, welcome to the Board.

You might want to remove this same question from the other thread so that that one can continue on its own course.

In answer to your question, I've never heard of a specific term for that situation in the English-American language.

I think she should stick with mother-in-law if her relationship with her m-i-l is still strong. It'd be cruel to be constantly bringing up her dead son.

And really, what is wrong with saying mother-in-law? Especially these days, where kids routinely have four grandmothers and grandfathers because of divorce and reconstituted families.

If somebody asks for specifics, she can always provide them.

If she likes her mother-in-law, she can also rest easy with the idea that this woman will probably feel honoured that she's still being referred to as mother-in-law even if her son is dead.