Firstly NO-ONE is "registered" with IMCA. IMCA is an association set up by some companies, they are not law, they do not "certify" anyone, they do not authorise anyone.
Who has appointed him as the Supervisor, the company that owns the ROVs? If so, it is up to them who they appoint and put in charge of their gear. If he has appeared onboard as Supervisor and has come from an Agency, I would suggest someone gets onto the ROV company and tell them to inspect his CV and paperwork from the Agency as someone has most likely lied or forged CVs or paperwork somewhere along the line.
However, it may also be that the ROV company have decided that they wish this person as a Supervisor on their equipment. If so, it is up to them.
If he does anything that endangers other personnel then I would suggest Stopping The Job.
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