Fi checks objective ethic with subjective ethic. Whereas Ti+Fe would observe objective ethics with a critical perspective, Fi+Te observes objective ethics in accordance with inner harmony. Te sees what is good/bad with what is widely accepted as what makes sense. Fi would override this by deciding which goods and evils feel right, and rejects those that don't. I guess.
a person can't come to creating his or her ethical principles without both functions together. Each create a balance. Pure Fi ethic would be too personal and might hurt others, and pure Te ethic is unquestioning and may hurt the self.