dlcm...
To place blame anywhere but on the pilots is fundamentally wrong. This is not an "AB" issue it's a question of basic airmanship. The PF took a perfectly functional airframe with a known manageable issue and fubar'd it so badly it was probably unrecoverable by the time the captain returned.
All the dribble about course change, rest etc is just that. The failure to fly "pitch & power" when confronted with unreliable airspeed is inexcusable for any licensed pilot...let alone one with an ATPL.