This is actually a tricky piece to understand, because it's not only the slang language of criminals, it's a metaphor in the slang of criminals.
Some criminals specialize in stealing goods - goods that are usually protected, in a location that is believed to be secure, such as protected inside a locked safe, inside a bank vault.
To accomplish such a theft, obviously the first obstacle that must be solved is getting inside that secured location. This is so common a problem that to have solved it is just abbreviated to "[being] in", or just "in". "Once we're in, I'll be the lookout, while you crack the safe." "Once in, we'll only have twenty minutes before the security system resets and the alarm goes off."
In the passage you quoted, they use slang of con artists such as 'mark' (the intended victim of a con artist's scheme) but they don't put it in quotes. They put "in" in quotes because it's being borrowed from a different slang, that of thieves, and used metaphorically. A scammer who convinces Grandma that he is actually her grandson has overcome the first obstacle in his crime, just as a thief who has broken into a vault has overcome his first obstacle. The thief is now in; metaphorically, the con artist is also "in".