Many have asked why the PF continued to pull back as the plane descended since it clearly indicated a stall. Maybe the FO disregarded the Altitude/VS data as well. Consider this:
He almost certainly would have disregarded any airspeed indications, so he's already thinking there's a pitot problem. Next thing he knows, the VSI varies between +7000 and -10,000. He's never seen those values before. Could he have assumed that the entire pitot-static system had gone bad?
The poor fellow now resorts to his last instrument- the attitude indicator. When he pulls back, the stall warning goes off, release the back pressure, and it returns. Catch 22 in his mind. Even if he knew that the stall warning would disappear below 60kt, if he had any idea that his airspeed was indeed below 60, he would probably have reacted more appropriately.
They probably put it all together when the RA started functioning and gave a "TOO LOW TERRAIN" alert, but at 10,000fpm, 15 seconds wasn't nearly enough time.
It must truly have been horrific in the cockpit. Flying in IMC, and none of your instruments or control inputs making any sense. RIP.