IIRC the Deutche Bank building had to be demolished due to the fact a large chunk of masonary gouged a gash down the front of the building and destroyed the interior. It seems likely that the surviving tower would suffer in the same way-requires demolishing but survives the actual attack.
The effect on the attack itself is intesting. We can already posit that the unhit tower would be as succesfully evacuated as the other buildings were (although some structures may not be evacuated if they are far enough away, it seems likely that the entire complex would be evacuated, possibly unofficially when people leave the buildings en mass to try to escape the possible area of debris.) We can posit a death toll between 1,000 and 1,500 (with 1,000 to 1,500 further people surviving) possibly lower taking into acount a less urgent evacuation.
One thing which does make me wonder though is this: Would it be possible for some form of zipwire or rope and pulley system to be set up between the surviving tower and the upper floors of the burning tower. Possibly some system whereby several ropes are strung across with instructions to tie them securely to a pillar or something, followed by harnesses to allow some trapped people to be evacuated from the upper floors. Might not work/be too risky though.