Well it is a very dificult question if we don't know what phase of flight this is.
For landing, tell the captain to go-around. If he doesn't tell him again. If he then doesn't go around, take control of the aircraft and do it yourself. And get the tapes pulled on the ground.
A go-around might be expensive, but if there is enough fuel... its probably the safest course of action.
In other phases, ask him what he is doing!