Mrs. Sadie Adler, Widow - I
Sadie needs Arthur's help clearing out the last of the O'Driscolls.
Prerequisite: Goodbye, Dear Friend
Gold Medal Requirements
For this mission, you must first complete Goodbye, Dear Friend and then have only one other mission available to you. It doesn't matter which mission this is, but this one will not trigger if there are multiple other missions to be done. Sadie will be waiting at camp afterwards, and once you talk to her she'll ask for your help. She wants to go and kill the last of the O'Driscolls, and she knows where they are hiding out. You can either choose to help her or refuse, and after some additional dialogue, this mission will end either way. If you do choose to help, she'll offer to ride with you ride now if you want or to wait for you at the spot if you want to come later. It is best to just ride with her, as the spot she's going to is all the way on the other side of the map and this will save you the hassle of getting out there yourself.
Source: Suprak the Stud's Red Dead Redemption 2 Guide