We would not normally expect a claimant being reassessed before the reassessment date on the ESA computer system, however, there is nothing stopping the DWP from doing so if they wish and the claimant cannot challenge the Decision by any of the normal legal procedures such as a Mandatory Reconsideration or appeal.
Please do not take my use of words to suggest that there is an abnormal procedure that could be used.
Even a complaint to the DWP will be of little value as it will be treated as a wholly separate procedure from the reassessment and will not remove the requirement on the claimant to participate in the process.
Their best bet is their MP but there are no guarantees that they will be able to remove the requirement.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems