Oh god, you absolutely need to backtrack here. First, it's a GRADE 4 cancer, not stage, that means something different.
I recently lost my husband of 41 years to a Grade 3 brain tumour. His prognosis was 3-5 years if you're lucky, 12-18 months if not (with that latter scenario meaning it would have become a Grade 4). He lived nearly 8 years. On the Brain Tumour Charity support page, there are indeed many people who die within months or years with a Grade 4. But there are some who live several years. It depends on so much.
For many people, the first choice of chemotherapy - temazolomide - is reasonably well tolerated. It's pills not intravenous, usually a few days per month. For my husband, it just made him very tired in week 3. Nothing more. Radiotherapy is usually soon after surgery, daily for a few minutes for some weeks. Again fatigue is the main consequence.
People wouldn't have known there was anything wrong with my husband except that his speech and memory were hit, because of the tumour location. There's a helpful gif showing life with a brain tumour, of an iceberg in the sea. Top bit is labelled something like "You look fine" and then the 90% below labels all the hidden side effects and consequences.
So honestly, unless you've got solid evidence something is amiss with her story, you have to drop this. If anyone had judged or suspected my husband as exaggerating or faking his illness, it would have absolutely broken me. And it was hard enough to deal with all the people saying "Ooh he's looking great! Doing so well!" and I wanted to say "you haven't a fucking clue"