1) Say 'go-around', loudly and clearly
. What else can you say unless your man has indicated he has adequate visual reference and can make a safe landing from the approach? At very least, if something untoward happens, the CVR will admonish you as F/O from any blame.
Personally, as a very junior co-pilot, if after the event, assuming nothing really unpleasant occurred, unless the Capt apologised fulsomely, saying he made a mistake and would never do it again, depending on the exact circumstances I share my concerns with a trusted person who was much senior to me, for example a friendly Training Capt, possibly initially at least deidentifying the Capt concerned with the incident.
Can anyone more experienced comment on what I've said?