I disagree.
This is a neutral act.
It is good (and maybe suicidal) to reach down and help the dwarf.
It is neutral to survive by pushing down the dwarf. The dwarf is already a goner shy of any other intervening forces. Neutral people tend to be pragmatic.
It is evil to push the dwarf down with your spear (and hence damage the dwarf and prevent him from grabbing your legs) to survive.
The dwarf is already submerged in a killing environment. Pushing him down further (when he is already probably 3 or more feet under, we do not know the race of the female PC) does not really do much to ensure that he still dies. There might be an argument that if he is too far down, nobody can save him. But, the PCs probably do not know that for sure and this is something that might not be thought of quickly in a desperate emergency situation.
Additionally, by pushing (presumably standing on) the dwarf, as a DM, I would have given the Dwarf the chance to grab onto her legs. In desperate situations, people do desperate things in order to survive. This applies to both the original PC and the dwarf. This also might have enabled everyone to be pulled out and survive.
Would you have ruled that it was an evil act for the Dwarf to grab her legs when she stood on him because he might drag her under?
Good for the goose, good for the gander.
Given that we have no other information to base this on, if the DM just based it on this, he overreacted. This is not like she just didn't like the dwarf and did an evil act of slitting his throat. She was struggling just to survive.