I'm afraid that there may not be a lot of places to go here.
The Council and/or the Cemetery Trustees are probably right.
As a general proposition, and subject to any contractual terms
(such as to design, or content), between the operator of the cemetery
and the person who erects* it, a headstone belongs to the person who erects it.
Which means, that person can pretty much** engrave
whatever words they want.
Consider also the possibility that the funeral sales rep might have said something
in the ballpark of "this is what's right and proper", and the widow may have been influenced by that.
Or maybe, she picked the words out of a stonemason's catalogue, or got them off the internet.
You'd think it was the executor who had the final say over this sort of thing.
Mostly though, people's funerals occur before Probate (or Administration)
is granted, and so before the executor/ administrator takes office.
So, often, it's the spouse, or other close kin, who make decisions
about funerals and any grave.
While I recognise how distressing this must be,
I don't actually see where you have any kind of
enforceable right to be consulted about,
let alone exercise any kind of veto over,
the words on a person's headstone,
just because you may not like them, or
disagree with what they say.
---------------------------------------------------
* That is, the person who arranges/ orders/ pays for a headstone,
even if they don't actually manually handle it into place themselves.
** Subject to the usual operation of other law, such as non-publication orders,
defamation (of anyone named on the stone who is not the deceased, that is),
offensive behaviour, obscenity etc