From what you wrote, her condition of "Fits or convulsions" you refer to is most possibly related to the brain tumor she had. I assume there's no history of epilepsy.
Even with epilepsy, data on the millions of COVID-19 vaccines administered so far does not show any link between the vaccines and seizure disorders. Many EU countries studied AZ, Pfizer and Moderna vaccines and found it's generally safe. Studies done on epileptic patients showed that with small percentage the frequency of fits increased for a few weeks. Also there isn't a preferred vaccine for people with "Fits" - all these accines behave the same.
But note there were allergic reactions to all these vaccines with some of the people.
People with brain tumours under treatment and those recovering (post-operative) do get vaccinated too.
However without knowing the medical history or the current medications she may be under no one can give you a definitive answer. The best is to contact your specialist who treats her and follow his advice.
You can't find fault with the vaccine centre staff. They acted correctly under the circumstances.