Ouch, this one sounds like a real toughie. During the flight itself i would try to make some sort of comment, even if quietly so something is recorded on the CVR, to cover yourself at the very least if the worst was to happen (let's hope not).
Next i would talk to a senior captain at my own base ( a TRI/TRE or even your base captain) or any other captain that you feel you could confide in.
Next up contact fleet captain, chief pilot or flight safety department.
If you have one, contact your union rep or call your union's legal department
finally if none of that works contact your local aviation authority. They can demand FDM recordings if they have a problem with what's happening.
Whatever you do, DO NOT STAY QUIET, get something done about it!!!!