I have worked with 2 people who did this.
One was by proxy though. She started work at my office and her mum died within days, then her partner got ill, then he had heart attack, then he had cancer, then he had a massive heart attack, then her dad died, then her partner had another massive heart attack and then he was so ill the packed her off from hospital with a their own personal defibrillator and it was her job to bring him back to life. The. Her mum does again. All in 8 months.
She was actually an alcoholic. These lies were to justify time off so she could sit at home getting pissed.
The other was purely about getting paid time off. She had a baby and returned from maternity leave a few days before I was going on mat leave. She told me that I should take 6 months may leave (full pay) come back for a couple a weeks then go on sick and get another 6 months off as our company gave 6 months sick pay.
I laughed and said I had saved for Mat leave and I was fine. Sure enough it's what she did. She would return to work for a few months then go on sick again.
The illnesses she had just got worse and worse until it was cancer. She had a stomach operation of some sort and the stitches got infected and she nearly died. then she turned up at work the next day in skin tight jeans to get something out of her locker. Tight across the stomach where the infected incision was. They had only revived her the day before.
It became more and more obvious she was lying then someone heard she had set up her own beauty/massage business, someone saw on FB that she was advertising treatments on FB. Someone took a screen shot and sent it to management. She was fired. The union even struggled to help her out and they wee usually excellent.
I kept out of it. Completely. I knew it would come to a head. It made my life harder, as I was a senior member as we couldn't replace her permanently so everyone else was having to cover her work or I would borrow someone from another team. But they weren't fully trained. Her sickness was looked after by my managers so I left them to it.
If she is lying OP, I would stay out of it. It's likely that it will increase and probably happen again.
If she isn't, things will settle down.
Unfortunately some people do lie and there are a range of reasons.