This mission in Red Dead Redemption will be available only after completing the quest «Lucky in Love». Head to the question mark marker to start this quest.
You will see a man named Mario beating Eva. If you kill him, the woman will run away in fear, and you will fail the quest. Instead, give Mario $200 to let the girl go. Thanks to your information, the girl will move to the church in Las Hermanas. Follow the point marked on the map to find her again.
The nun will tell you that Eva ran away with some man to Sepulcro. Upon arriving at the nun's landmark, you will see Mario burying Eva's body.
Of course, we cannot tolerate this, so we challenge the man to a duel and kill him using the «Dead Eye» ability.