Stand up against Sam (the dog who owns the diner) at least twice.*
The first one is on the way to Sam's Diner to talk to Mary for the first time (while he's in the car):
Respond with "You watch it"
The second time is after Mary's shift has ended and she took the bus home:
Head back to the diner and Sam will be in her stead, he'll be annoyed at you and threaten you
Select "try mr eat my fist" and he'll back down and offer a free hamburger.
You'll give the burger to the goat which will also give you the Good Samaritan Achievement
There is a third opportunity to get this achievement if you said "you watch it" with the first interaction. This happens after you find the file behind the painting in Joe Dunn's apartment and go to Sam's Diner
After you pull of the guise of a health inspector Sam will question why his interrogation had anything to do with food safety and say you're not a real health inspector.
Respond with "So what?"
You don't get the hamburger this way
*The third way was how I got the achievement and I had no idea the second option was there until another playthrough and when I looked through a few forums and guides.