This question pertains to the principles of natural justice and fairness in administrative proceedings. The correct answer is 'Allow them to rejoin the proceedings'. If a party can demonstrate 'good cause' for their absence (e.g., illness, unavoidable emergency), fairness dictates that they should be given an opportunity to present their case, and the ex-parte order may be set aside or they may be allowed to participate from that point.
'Fine them heavily' is punitive and inappropriate when good cause for absence is established.
'Dismiss their case' would be unjust if a valid reason for absence exists, denying them a fair hearing.
'Refer them to the Board' is an unnecessary procedural step; the Hearing Officer typically has the discretion to manage such situations.