From Red Dead wiki:
"Marston encounters a strange man south of Mescalero, who seems to know a lot about Marston's past and claims to know him from his days running with the Dutch van der Linde Gang. However, the strange man is very coy and never reveals how he knows Marston.
He then asks Marston to complete a couple of tasks, both of which turn out to be moral dilemmas. And the Strange Man's tone hints at what course of action he believes Marston will take in each case."