Your friend is lucky, I know several people personally who have been dragged through the courts for months before they were able to get their incorrect assessment overturned.
The stricter regime for both ESA and JSA has created a dreadful gap in the benefits system - people who aren't sick enough to claim ESA, but too sick to claim JSA. Many people with health conditions that vary over time are refused ESA because their condition is quite mild at the time of assessment, then repeatedly sanctioned on JSA because they become too sick to keep up with their jobseeking activity obligations. There appears to have been a particularly severe impact on people with mental health conditions and learning disabilities.
Our benefits system has become overly complex and extremely unforgiving. A relatively minor error can leave someone completely destitute. The services that help people navigate the benefits system are under severe pressure due to austerity cuts. I'm no bleeding-heart liberal, but I think that our benefits system has become callous, unreasonable and ineffective in preventing poverty.
As I said it's not exactly under-reported, that was your cue to search and find something like: https://fullfact.org/news/how-many-fit-work-assessments-are-...
And (to address your other comment) I assume you aren't being literal with your specific example (because the guy you were responding to didn't literally mean people were assessed on their ability to walk to a shop specifically), but yeah there have been reports of people who can't walk very far getting told they're fit for work (For instance this guy: https://www.cornwalllive.com/news/cornwall-news/photographer... from earlier this week)
Where's your evidence?
http://www.independent.co.uk/news/uk/politics/dwp-disabled-p...
https://www.theguardian.com/politics/2018/feb/12/disability-...
Happy now? Or are you perhaps going to try to play the no true Scotsman PR game and claim this doesn't count as real evidence?
If the guy had replied saying he actually did know the person I would have to accept that as true. He didn't.
Despite recent evidence, this isn't The Guardian and I am willing to believe that most people here are honest and don't just make stuff up to suit their political agenda.
To answer your question, two lefty news articles and fakenews.com don't count as evidence, no. His word would have been good enough for me.
Granted: Nobody’s linked to any papers or studies. But neither have you.