Both Marshall Island and Vanuatu Ship Registers are actually operated from USA by Americans, as Is Liberian Register. Not that that help you anything on the legal aspect, but it makes it easier to obtain contact and no language problems.
Since both MIR and LIR are among the biggest international registers and cherish being on the MOU Whit List, they may be more willing to look into this case than Vanuatu, which is mostly used by American OSV and Drilling companies and not on the White List.
Class has nothing to do with wage disputes. They do check and issue ISM Certificates on behalf of flag states, but has no other involvement, or interest in enforcement. ONLY the flag state has that right.
ILO is the organization behind MLC’06 but, like IMO, they have no enforcement function.
ITF sets minimum wages for seafarers, but I assume your Belgique friend will still be paid above that? They do have enforcement capabilities through member unions around the world, but how strong the unions stand in Brazil, and whether they are interested in getting foreign workers out of their patch is an open question. (Would US Unions do anything for foreigners on rigs or boats in the GoM??)
The last thing. Who actually own the rig and who did the workers sign a contract with? In most cases you will find that the actual Owner is a paper company in the country of register, whatever.
That company is usually 100% owned by a Holding company in Delaware, or in BVI, CI, or some other convenient place.
Many times the crews are hired through a Crewing Agent, who hold their contracts and is ultimately responsible for their payment, repatriation etc.
I obviously don’t know if any of the above applies in your case, but is may be worth checking before getting into a legal battle. Maybe a better solution is for all crew members get together and threaten to resign, unless any drop in wages is an agree upon flat rate.
A little anecdote from way back in time, when the Toolpusher was next to God on US rigs.
On a rig working off Brunei with an Iban crew, a new Toolpusher came fresh from the GoM.
He got all worked up when one of the Mess boys showed up in the recreation room all dolled up for the movie and wanted to fire him on the spot. When all the rest of the crew downed tool and refused to go back to work until the Toolpusher was removed, the Management quickly decided that it was cheaper and better to replace one person than the whole crew. The replacement Toolpusher got a lecture on Iban culture before joining.