Refusing to rescue Tarquin was a purely Neutral act -and would have been so even if Elan had expected his father to die from the impact. Inaction is, after all, the ultimate neutrality. (Neutral is not Nice; just take a look at the animal kingdom.) Besides, Tarquin got himself into that predicament as a direct consequence of deliberate Evil acts; falling to his death would merely be reaping what he'd sown.

Now, if Elan had taken pleasure in watching him fall, that is another matter...