Here's why:
* Parallel Structure: The two parts of the sentence ("he needs to finish the project" and "she does") are grammatically parallel. They both use the same verb structure (needs/does).
* Conjunction: The conjunction "either...or" is correctly used to join the two clauses, indicating a choice.
However, the sentence could be improved in terms of clarity. It's better to explicitly state what "she does." For example:
* Either he needs to finish the project, or she needs to finish it.
* Either he needs to finish the project, or she needs to do it.
This avoids ambiguity and makes the meaning more clear.