After PIP 5 years+ Demand for Sick Notes

My daughter has been on PIP some years now and received the initial award as DLA following tribunal. She had a massive hernia and no operation done. Following move House she found herself pregnant after being told before she moved she wasn’t. This didn’t surprise her as she had sistolic ovaries and unusual periods her whole adult life. So in new area, still with this major hernia and pregnant she was sent to hospital. Assessment done and eventually major op done at same time as ceasar delivery. Her health improved and 12 weeks after op she was the best she had been however the consultant emphasised the op had input a protection netting (as they used then) and that she would never be able to do any heavy physical work, she suffers depression from symptoms of systolic ovaries, heavy periods if any, she is dyslexic and if Kate high blood pressure and type 2 diabetes. She is due PIP review next year.
However the job centre has insisted she now supply sick notes as her child is now 5, she has told them she is in PIP and cannot work due to her health problems but they insist she supplies sick notes again. Under threat of benefit being stopped.
How can this be right, she has been under tremendous stress and has also recently been in hospital with pneumonia.
Is it because she’s working age, or that her son is 5. She was released from consultant and although the op mended the Hernia she still has other health indicators as in first PIP award pkus others.
What can she do?