The FO sitting in the jump seat was probably too scared to speak up about the
loss of Airspeed if he indeed even noticed it.
Lets imagine he did speak up and saved the day, how do you think the two Captains would have reacted later on? I think the FO might have been told off and probably this would have effected his career later on. So he probably saw the low Airspeed and "thought" they had it under control.
I can categorically say this wouldn't be the case in my outfit, our relief guys have spoken up in the past and indeed saved a lot of heartache...
That's what they are there for.