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.
It was incredibly unprofessional of him not to say something if he spotted it, which he should have. He claims that he couldn't see the runway which is understandable from a jump seat on late finals even if they had been on the correct profile. Therefore what else did he have to look at apart from the instruments?
This looks like a classic and massive failure of CRM.