After completing Good, Honest, Snake Oil in Chapter 2, you will find the sheriff's office in Valentine has a new wanted poster on the wall. Grab the poster and if you want, you can question the sheriff to learn more about what Mrs. Swan did. However, as soon as you get the poster, a marker will be put on your map, showing you the general location of Ellie. Note that this bounty needs to be brought in alive, so you can't be killing her or else you'll fail.
Ellie Anne Swan is located a little southwest of where Cumberland Falls is marked on your map. She will be on the western bank of the river there, arguing with her current boyfriend. Approach them to have Ellie try to send her boy toy after you, but if you don't do anything, he won't do anything either. She will off him, then come charging at you with a knife in her hands. Remember that she needs to be alive, so get your lasso out and quickly rope her, then hogtie to incapacitate her completely. Load her on your horse, then bring her back to the sheriff's office in Valentine to finish the bounty. Return to the sheriff's desk to receive your reward ($25).