It is God who decides which dua is to be accepted depending on His wisdom and planning. No evil prayer harms a true believer and no good prayer can protect an insincere believer.
Sometimes God gives opportunity to disbelievers/wrong doers so that he has no excuse left that he was unable to reform himself, so he continues to do wrong until he goes to Hell.
If oppression took place between you two, it is possible for evil prayer to be accepted provided sincerity is there and the outcome is deemed good for you in this world and hereafter. A true dua is when my thoughts, my heart and my actions are in synchrony then the dua is accepted.
Evil prayer is not good manner for a believer. We need to rely on Him because He is always with the oppressed and patient ones. Taking revenge can be your right but it is best to leave on Him to take action. He is احكم الحاكمين meaning the Best of the Judges. We as humans can make a mistake in assuming. Criterion for decision making should be Quran (the principles in it). He has promised that when you rely on Him on all matters, he will reform for you everything, whatever you need. و افوض امري الي الله انّ الله بصير بالعباد “wa ufawwidhu amree ilallah, innallaha baseerum bil ‘ibaad…..”
Saqalain Abbas Alavi