A relative has a bottle of inexpensive champagne (Charles
> Courbet) which has been stored in a kitchen cupboard for 3-4
> years. Is it likely to have deteriorated
Definjitely yes.
to the extent of being undrinkable?
That very much depends on the definition of "undrinkable". While it seriously will be way over the hill (and won't deliver much pleasure), it certainly won't be toxic.
She was considering giving to someone as a present but doesn't
> want to risk giving offence if it's no good.
I'd refrain from doing this.
Neither of us know much about alcohol.
"Alcohol" - at least here in Europe - normally would refer to destillates, which wine is not.
M.