I believe you, but I also think it would be hard for you to prove if she has medical evidence to support her case.
I have a relative whom I can't prove is making stuff up, but whenever money is tight or they no longer want to work they suddenly develop illnesses that are often diagnoses of exclusion where there's no singular test that can disprove illness. All her symptoms seem to manifest usually at work, after she's spent weeks moaning about how she's fed up with the job and just wishes she could quit but she's got no money and how it's not fair that she has to pay for her car when those benefit scroungers get their cars for free.
The issue is that you're up against a paper trail of evidence for illnesses that are not consistent, and when they decide to give you an award they look at the evidence and then read what you've put and they make a judgement based on whether what you've written is plausible with the evidence you've provided.
I can't disprove my relative has FND for example, although she's here most days and never has any symptoms, because she isn't here all day most days and it's when she's not here that she says she experiences them.
At the same time my FIL is very open about the fact he needs to make a doctor's appointment regularly to complain about the side effects of the medication they prescribed him last time having such adverse effects, despite the boxes being unopened in his cupboard, and how he needs to ask for a referral to yet another specialist knowing they will say his case is too complex, simply to build the paper trail. He even marks them in his calendar ahead of time so it looks reasonable. I HAVE reported him, I've provided photo evidence and written statements, I've written to his GP, and he has been investigated but because the evidence outweighs what I am saying, like his manufactured memory issues and saying "oh I just forget that I've already got a box of XYZ, I need help managing my medication and nobody is helping me" it goes no further than a preliminary call.
I've been asked to help my BIL apply for PIP, who appears to need it, but I've asked for his supporting evidence so I can build the best case possible but he's fucked off to Uganda to see his girlfriend, where he appears to be disability free and his supporting evidence has not been forthcoming so I have categorically said no. Being disabled isn't an automatic entitlement to PIP, and if he can travel solo halfway across the planet and cook, clean, shower, eat and manage his time for weeks then he does not qualify and I feel his issues are exacerbated at home through learned helplessness.
The issue is that if you don't understand how these conditions manifest, you couldn't reasonably challenge it, and if you do understand how they manifest and it rings true, you may even change your mind about challenging it if you find out something you didn't know before.
As I said earlier, I am on benefits and I know that some people could look at me and not see my disabilities or how I am supported to look and feel as "normal" as possible, and them not see all of the work that goes into making normal every day outings and tasks possible for me, so I like to err on the side of caution because we can't possibly know everything about someone unless you're extremely close, but a lot of what you say rings true about my experiences with other people who are on or applying for benefits.
The fact is we're not owed anyone else's medical history, and it's jolly hard to prove fraud.