Decision based on fraud investigation

I’ve just picked up a case which is unusual and I really want to challenge DWP decision. Only problem is it was made 5 years ago.  I haven’t seen any paperwork yet. Have copied and pasted referral details :- 
The gentleman in question is
suffering from a severe form of Parkinson’s disease and he used to be in
receipt of a higher rate of DLA (mobility and care) due to the nature of the
illness and the way it can change from day to day.
The gentleman was at home one day on his own and he had a visit from a
representative from DWP and a doctor, who stated that they knew nothing
about Parkinson’s and they were very rushed and could not stay long etc,
etc.  They noticed a caravan in the road and asked how he managed to go
caravanning with his illness and he stated that the caravan was not theirs,
they pushed him further with questioning about the caravan and holidays etc.
They also queried that he had stairs in his home and how he could possibly
use them (he has had several falls down the stairs , also at night and does
not use them when he is in the home alone), he explained this, but he did
not feel that they believed him.  He reported lots of other line of
questioning that did not sound appropriate, especially as he is a vulnerable
gentleman alone in his home, with no support from his wife/carer.
Shortly after this visit his higher rate of DLA was stopped and he is now on
the lower rate and financially they are a lot worse off and they had to give
their mobility car back and buy another car, which they did not have the
money for, as they were making use of this to help get out and about, which
is very much needed as their location is rural.
The wife telephone the DWP and asked if they could appeal against the
decision and they said that it was not worth it as the doctor had visited
and made a decision so this would be a waste of time!

Sounds like a fraud investigation to me. I know appeal rights have run out but do you think I could get the decision revised because of official error? Looks like a lot of assumptions have been made, and appeal rights were denied by callcentre!
Any advice appreciated.

I would make a request to inspect the whole DLA file what you find there would determine the next step.

How long ago was the decision? If less than 13 months, try for a late appeal - nothing to lose, and if you explain he was “misled” into believing an appeal would not succeed you may well get it let in. Meanwhile, assemble the best evidence you can to support his case and send it in ASAP in the hope they will review it anyway.