How can one repent and redress the injustice done to a colleague who was lied to and corresponded with under the guise of an admiring young man, given the impossibility of contacting her and telling her the truth?

What you did was wrong, and you must repent and show remorse for this act. As for the right of the girl, increase your supplications and prayers for forgiveness for her. If informing her would lead to a predominant harm, there is no need to inform her. Some scholars have chosen that the backbiter should seek forgiveness and pray for the one they backbit against, in order to avert the harm of them being distressed by the disclosure.