Misrepresentation - decision making

What are the statutory rights / basis and or Civil Service Code or DWP policies for DM decisions? - Following a friend's FtT decision earlier this year which accepted good cause for non-attendance (which was effectively a misrepresentation by the assessment provider - similar to this advice here on B&W: benefitsandwork.co.uk/forum?view=topic&catid=10&id=107189) and confirmed LCW, a DM confirmed in writing placement in SG for 24 months before reassessment. Now suddenly, a few months after the DM's decision, the assessment provider calls for another WCA this month. After some misrepresentation by other staff and finally tracking down the DM who confirmed the 24 months, that DM also backs down and simply declares the earlier decision as an "error". Initially the DM calculated the 24 months from the date of the FtT decision. Now the DM fudged his more recent response by suddenly backdating the 24 months from any odd earlier date in the claim in 2013. According to DMG 42296:
"The DM should apply the recommendation of the FtT as to when the next WCA should take place, from the date of the original decision unless the FtT specifies otherwise." As the FtT hasn't specified it, hence it was at his discretion to apply the 24 months calculated from the date of the FtT hearing and there is nothing "wrong" about this, as the DMG allows for this.
Even if the DM had applied the 24 months from the original decision in 2016, 24 months would have still taken it to 2018.
How can this be addressed and what are my friend's statutory rights regarding this? Given the short time span before the WCA, a complaint would not help at all, also as they are notoriously ignored.

Thanks very much Gordon for the advice regarding the MP - hopefully they are available any time soon!

No ESA50 was sent by the AP and as the claim is based on MH issues, the 7+ days time frame between notification to attend and WCA is insufficient to fill out an ESA50 to take with them to the WCA when normally claimants would be given at least 28 days to return it!

Also the claimant is not native speaker and the AP hasn't arranged an interpreter yet - wouldn't proceeding with the WCA without an interpreter be a breach of the HRA - at least the FtT conceded at the previous hearing that claimants are entitled to one!

Re: "ESA awards are not for a specific period or to a specific date, no date is included in the Decision." - What is the administrative relevance then if claimants are awarded PIP, ESA, IB etc. or placed in the WRAG or SG group for x-amount of time (e.g. 12, 18, 24 months)? It appears to be entirely arbitrary government / decision making by civil servants then if this can be revoked without reason or indication of change of circumstances any time during that award?!

If the medical conditions have remained the same and the AP hasn't bothered to send out an ESA50, surely after such long time the information they hold on file cannot be interpreted as a current snapshot in time? Else how would the friend request a filework based review prior to a WCA?

It was mentioned here or elsewhere that new rules regarding reassessments have been introduced recently - to save long-term chronically ill claimants from repetitive WCAs - how do they effect reassessments in practice?

Re: "they set a reassessment date" - well, maybe my wording previously used wasn't adequate - I meant exactly what you stated i.e. that DM set a reassessment date (well, the DM phrased it differently i.e. "placed in the SG from date a - till date b" and added: "as a result [the claimant is] not required to attend any assessments during this time).
So what does that mean in practical terms, especially as the same DM now claimed he stated the above period in error - i.e. can they always get away with their maladministration by simply labeling it as an error?