Pip help for a family friend who has long term disability

ilovepink97

hi so a family friend is in the mandatory reconsideration phase we realised she gets a low rate of pip and has been for all these years she has no disputed this ever but she’s been getting the lowest payment for mobility and dialy living ever since her surgery this was when she became too sick to work so she applied for pip (this was about ten years ago)

dwp had wrote her a letter saying from when did her health condition worsen but my point is it’s always been really bad but she hasn’t been paid properly for almost ten years

my question is can pip be backdated this long ago and what should I advise the family friend as she’s really anxious