I’m very for your loss and for how terrible it must be to see that account used by someone else.

When you say that Facebook hasn’t been helpful, does it mean someone from her family contacted Facebook through the memorialization or removal process and that didn’t work? This process and request are documented here: https://www.facebook.com/help/contact/228813257197480

If Facebook hasn’t helped after this request, then here’s an upvote to find someone in Facebook who can help.

Will try this route thank you now that she has died.

Facebook is fairly automated. A lot of stuff that's extremely clear to a human doesn't get taken down. One trick that seems to still work is mob reporting. It works as an attack, but also works with scammers.

FB always says that it doesn't violate their community standards, then the account disappears a day later.

There's a Meta service called oops. I don't know anyone important enough to use it. It's only usable for family and friends, not strangers per se or de jure.