Many cabin crew appear to be under the impression that it is their discretion. I was once told by a male CC that there was "no way" he was going to use his discretion. I left him in Muscat and he was later sacked too.
It is your discretion as the commander. However, in the UK, you are not allowed to force someone to operate if you believe them to be fatigued. If they have just told you they are, you are in a bad place to say "You're working".