Her name is Charlotte Morgan.
She Tweeted about it.
This I think is the important part. Confirms that it's an iPhone she uses.
Now I don't have a Santander account so unsure how the view pin function works, which is something she's very hung up on in her Twitter thread but;
Someone would need to unlock her phone (via pin or biometrics), then unlock the banking app and pass additional auth to view pin of the card.
Of course that someone has already seen her pin, at a cash point, in a shop, at the gym barriers (which were not working at the time) or maybe just possibly on a piece of paper in her bag. And this pin to unlock the phone is the same she uses for her cards and at the gym barriers etc etc.
But then she says things have been 'reset'.
But in order for them to do that they'd need to know her personal ID number for online banking, does that show in the Santander app if you can access it? If not then they'd have to do a 'forgotten my ID' process on the website which requires at least your full name, D.O.B and postcode. Then I assume it asks for more security information.