I Know You

 is a mission in Red Dead Redemption.

Story
You encountered a strange man who seems to know a lout about your past and claims to know you from your days running with Dutch van der Linde. He asked you if you would go to Thieves Landing to check on a friend of his, who is drunk and thinking of cheating on his wife. The man did not explicitly say to stop him, just to advise him on how to best proceed.

Walkthrough
After meeting the mustached stranger, head to Thieves Landing and go to the saloon. The drunk friend should be on the porch.

You can either dissuade the drunk friend from cheating on his wife, or give him $15 to persuade him to cheat on his wife.

There are likely 2 more interactions with the mustached stranger during mission.